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| United States Patent |
5,418,802
|
|
Chwalek
|
May 23, 1995
|
Frequency tunable waveguide extended cavity laser
Abstract
A frequency tunable waveguide extended cavity laser formed with a laser
diode, a channel waveguide comprising an electro-optic material, a
frequency tunable Bragg grating reflector formed in or on the channel
waveguide and a pair of conducting electrodes. The frequency of radiation
produced by the frequency tunable waveguide extended cavity laser is
adjusted so as to be at a desired frequency by applying a voltage to the
conducting electrodes. The device further incorporates a waveguide
nonlinear optical frequency converter to produce frequency convened
radiation at a higher frequency. Feedback means are included for
dynamically controlling the desired frequency so as to be at a frequency
for optimum frequency conversion.
| Inventors:
|
Chwalek; James M. (Rochester, NY)
|
| Assignee:
|
Eastman Kodak Company (Rochester, NY)
|
| Appl. No.:
|
152559 |
| Filed:
|
November 12, 1993 |
| Current U.S. Class: |
372/20; 372/22; 372/32; 372/64; 372/96; 372/97; 372/99; 372/102 |
| Intern'l Class: |
H01S 003/10 |
| Field of Search: |
372/18,64,22,99,21,96,97,20,102,32
|
References Cited [Referenced By]
U.S. Patent Documents
| 4667331 | May., 1987 | Alferness et al. | 372/12.
|
| 5119393 | Jun., 1992 | Oka et al. | 372/20.
|
| 5170402 | Dec., 1992 | Ogita et al. | 372/26.
|
| 5175681 | Oct., 1992 | Aoyagai et al. | 372/20.
|
| 5177758 | Jan., 1993 | Oka et al. | 372/102.
|
| 5185752 | Feb., 1993 | Welch et al. | 372/96.
|
| 5220578 | Jun., 1993 | Koch et al. | 372/96.
|
| 5253314 | Oct., 1992 | Alferness et al. | 385/40.
|
| 5291512 | Mar., 1994 | Suzuki | 372/99.
|
| 5301201 | Apr., 1994 | Dutta et al. | 372/20.
|
| 5311540 | May., 1994 | Pacholle et al. | 372/97.
|
| 5325392 | Jun., 1994 | Tohmori et al. | 372/96.
|
Other References
R. W. Tkrach and R. Chraplyvy, J. Lightwave Tech, LT-4, 1655 1986.
Zernike and Midwinter, Applied Nonlinear Optics, Wiley, New York 1973 Chap.
5.
|
Primary Examiner: Scott, Jr.; Leon
Attorney, Agent or Firm: Owens; Raymond L.
Claims
What is claimed is:
1. A frequency tunable waveguide extended cavity laser producing radiation
at a desired frequency comprising:
a) a laser diode having a reflecting rear facet and a front facet for
emitting radiation;
b) a channel waveguide for receiving radiation from said front facet of
said laser diode, a portion of said channel waveguide having an
electro-optical material in the waveguide channel that undergoes a
refractive index change in response to an applied electric field;
c) a Bragg grating reflector formed relative to said channel waveguide
portion having said electro-optical material; and
d) means for applying an adjustable electric field to said Bragg grating
reflector to change the refractive index of said electro-optic material so
as to be at a desired frequency.
2. The device of claim 1 wherein the front facet of said laser diode is
coated with a partially reflective coating.
3. The device of claim 1 wherein the front facet of said laser diode is
coated with an anti-reflective coating.
4. The device of claim 1 wherein said Bragg grating reflector is composed
of said electro-optic material.
5. The device of claim 1 wherein said Bragg grating reflector is composed
of an optically transparent inorganic material positioned on the surface
of said channel waveguide.
6. The device of claim 1 wherein said Bragg grating reflector is composed
of an optically transparent organic material positioned on the surface of
said channel waveguide.
7. The device of claim 1 wherein said means for applying an adjustable
electric field to said Bragg grating reflector comprises an adjustable
voltage system and a pair of conducting electrodes which surround said
Bragg grating reflector.
8. A frequency tunable waveguide extended cavity laser producing radiation
at a desired frequency comprising:
a) a laser diode having a reflecting rear facet and a front facet for
emitting radiation;
b) a channel waveguide for receiving radiation from said front facet of
said laser diode, a portion of said channel waveguide having an
electro-optical material in the waveguide channel that undergoes a
refractive index change in response to an applied electric field;
c) a Bragg grating reflector formed relative to said channel waveguide
portion having said electro-optical material;
d) means for applying an adjustable electric field to said Bragg grating
reflector to change the refractive index of said electro-optic material so
as to be at a desired frequency;
e) a nonlinear optical frequency convening waveguide portion having a
nonlinear optical material which converts the frequency of said desired
frequency to a higher frequency; and
f) detection means opto-electronically responsive to the frequency
converted radiation for continuously measuring the frequency converted
radiation power, and feedback means for dynamically controlling said
desired frequency so as to be at a frequency for optimum frequency
conversion.
9. The device of claim 8 wherein the front facet of said laser diode is
coated with a partially reflective coating.
10. The device of claim 8 wherein tile front facet of said laser diode is
coated with an anti-reflective coating.
11. The device of claim 8 wherein said Bragg grating reflector is composed
of said electro-optic material.
12. The device of claim 8 wherein said Bragg grating reflector is composed
of an optically transparent inorganic material positioned on the surface
of said channel waveguide.
13. The device of claim 8 wherein said Bragg grating reflector is composed
of an optically transparent organic material positioned on the surface of
said channel waveguide.
14. The device of claim 8 wherein said means for applying an adjustable
electric field to said Bragg grating reflector comprises an adjustable
voltage system and a pair of conducting electrodes which surround said
Bragg grating reflector.
15. The device of claim 8 wherein said electro-optic material and said
nonlinear optical material are the same material.
16. A frequency tunable waveguide extended cavity laser producing radiation
at a desired frequency comprising:
a) a laser diode having a reflecting rear facet and a front facet for
emitting radiation;
b) a channel waveguide for receiving radiation from said front facet of
said laser diode, a portion of said channel waveguide having an
electro-optical material in the waveguide channel that undergoes a
refractive index change in response to an applied electric field;
c) a Bragg grating reflector formed relative to said channel waveguide
portion having said electro-optical material;
d) a nonlinear optical frequency converting waveguide portion having a
nonlinear optical material which converts the frequency of said desired
frequency to a higher frequency and for which said nonlinear optical
frequency converting waveguide portion is located in between said laser
diode and said Bragg grating reflector;
e) means for applying an adjustable electric field to said Bragg grating
reflector to change the refractive index of said electro-optic material so
as to be at a desired frequency; and
f) detection means opto-electronically responsive to the frequency
converted radiation for continuously measuring the frequency converted
radiation power, and feedback means for dynamically controlling said
desired frequency so as to be at a frequency for optimum frequency
conversion.
17. The device of claim 16 wherein the front facet of said laser diode is
coated with a partially reflective coating.
18. The device of claim 16 wherein the front facet of said laser diode is
coated with an anti-reflective coating.
19. The device of claim 16 wherein said Bragg grating reflector is composed
of said electro-optic material.
20. The device of claim 16 wherein said Bragg grating reflector is composed
of an optically transparent inorganic material positioned on the surface
of said channel waveguide.
21. The device of claim 16 wherein said Bragg grating reflector is composed
of an optically transparent organic material positioned on the surface of
said channel waveguide.
22. The device of claim 16 wherein said means for applying an adjustable
electric field to said Bragg grating reflector includes an adjustable
voltage system and a pair of conducting electrodes which surround said
Bragg grating reflector.
23. The device of claim 16 wherein said electro-optic material and said
nonlinear optical material are the same material.
Description
BACKGROUND OF THE INVENTION
Tunable single-mode laser diode sources have potential uses in many areas
including spectroscopy, coherent communication, and nonlinear frequency
conversion. Particularly in the area of second harmonic frequency
conversion, also known as second harmonic generation or SHG, the narrow
band nature of this process dictates the need for a narrow band tunable
pump source which can maintain optimum efficiency as a result of drifts in
temperature, wavelength, or to compensate for device fabrication errors.
If these optical frequency converting devices (laser diode pump source
combined with a SHG device) are to be used as a means to convert invisible
infrared radiation to visible radiation as would be their use as the
source in optical disk recorders, printers, or display devices then light
output stability, compactness, and low cost are important additional
factors.
Standard commercially available single-mode laser diodes sometimes referred
to as Fabry-Perot laser diodes can be tuned by varying their temperature
or injection current or a combination of both. However, tuning a laser
diode by these methods often leads to undesirable effects. For example,
although a large range of tuning may be obtained by varying the
temperature of the laser diode the tuning will be interrupted periodically
by shifts in the lasing frequency. These frequency shifts or mode hops
would be detrimental to the output of a SHG system since the result would
be large drops in intensity of the frequency converted radiation. The same
detrimental result would occur for injection current tuning due to mode
hops as well as the fundamental pump intensity dependence on injection
current.
In recent years much research has gone into tunable single-mode laser
diodes which are free of these undesirable tuning characteristics. They
are generally of two types referred to as Distributed Bragg Reflector
(DBR) laser diodes or Distributed Feedback (DFB) laser diodes. Although
they differ in structure, in the broadest terms, their spectral
characteristics are a result of the incorporation of a grating structure
in the cavity. The laser diodes are tuned by an independent injection
current which modifies the index of refraction in the grating structure
via a change in carrier density as a result of the change in current. The
spectral and tuning characteristics of these laser diodes as known in the
art are well suited for the applications discussed previously.
Unfortunately, they are not as yet commercially available. Although they
may be commercially available in the near future their initial cost could
prove to be prohibitive.
An alternative to the DBR or DFB laser diodes involves the use of
commercially available low cost Fabry-Perot laser diodes. These laser
diodes can be made tunable through the utilization of an external optical
feedback means. It has been known for some time that optical feedback can
be used to control the oscillation frequency of a laser diode. (See R. W.
Tkach and A. R. Chraplyvy, J. Lightwave Tech. LT-4, 1655 1986). Devices
incorporating this effect referred to as external or extended cavity
lasers are well known in the art. They are typically comprised of a
Fabry-Perot laser diode with a frequency select portion of the emitted
radiation being fed back to the laser diode via a reflective diffraction
grating. The diffraction grating is often blazed and used in a Littrow
configuration where the first order is reflected back into the laser
diode. With enough feedback the laser diode will oscillate or "lock" to
the frequency selected by the diffraction grating. By changing the angle
of the grating with respect to the incident radiation it is possible to
change the frequency selected by the diffraction grating there by tuning
the laser diode. In some cases the front facet of the laser diode is
partially reflective. In this case the external cavity is a perturbation
of the laser diode cavity. In another case an attempt is made to
anti-reflection (AR) coat the front facet to eliminate any reflection from
this facet. In this case the cavity is incomplete. There is no feedback of
radiation from the front facet and the laser diode by itself will not
lase. The diffraction grating completes the cavity forming an extended
cavity. In practice it is impossible to completely suppress reflections
from the front facet resulting in some influence from the internal laser
diode cavity. In either case the tuning principle is the same. The
individual tuning characteristics differ slightly. For many applications
the extended cavity laser scheme is preferred.
The narrow spectral linewidth, low noise, and tunability exhibited by
extended cavity lasers make them attractive. Integration of the grating
into a waveguide is recognized as a method for possibly reducing the size,
weight, mechanical complexity and cost of the source. Such devices
employing channel waveguides and etched gratings in glass have been
demonstrated. (See D. A. Ackerman, M. I. Dahbura, Y. Shani, C. H. Henry,
R. C. Henry, R. C. Kistler, R. F. Kazarinov, and C. Y. Kuo, Appl. Phys.
Lett. 58, 449 1991). These devices involved an infrared laser diode source
only and were not used for waveguide nonlinear frequency conversion nor
did they include a means for frequency tuning.
A popular waveguide nonlinear frequency conversion technique for the SHG of
visible light involves the periodic reversing of ferroelectric domains in
ferroelectric materials to provide phasematching of the fundamental with
the second harmonic. This technique often referred to as quasiphase
matching or QPM is well known in the art. A device had been presented
which utilized a periodically poled QPM LiNbO.sub.3 waveguide for
second-harmonic generation in an attempt to provide automatic QPM by using
the periodically poled structure not only as the QPM element but also as a
Bragg grating reflector. (K. Shinozaki, T. Fukunaga, K. Watanabe, and T.
Kamijoh, J. Appl. Phys. 71, 22 1992). Problems with this technique arise
from the fact that the conditions for Bragg reflection and QPM are
distinct. As a result, impractical fabrication tolerances are placed on
the period of the periodically poled structure. Recently, a device
described by U.S. Pat. No. 5,185,752 to Welch and Waarts combines a
periodically poled QPM nonlinear waveguide, a Bragg grating structure, and
a means for achieving a TM polarization in the QPM waveguide section when
coupled to a laser diode. The TM polarization allows use of the highest
nonlinear optical coefficient in the most commonly used nonlinear optical
materials. The structure described by U.S. Pat. No. 5,185,752 to Welch and
Waarts does not include a device for frequency tuning.
SUMMARY OF THE INVENTION
It is an object of this invention to overcome the above drawbacks and
provide a low cost, compact frequency tunable waveguide extended cavity
laser which can be integrated with a waveguide nonlinear optical frequency
converting portion resulting in a compact waveguide frequency convening
extended cavity laser.
A further object of this invention is to provide a frequency tunable
waveguide extended cavity laser producing radiation at a desired frequency
comprising:
a) a laser diode having a reflecting rear facet forming one end of the
cavity of the frequency tunable waveguide extended cavity laser and a
front facet for emitting radiation;
b) a channel waveguide for receiving radiation from the front facet, a
portion of the channel waveguide having an electro-optical material;
c) a Bragg grating reflector formed in or on the channel waveguide portion
providing frequency select output coupling completing the cavity of the
frequency tunable waveguide extended cavity laser; and
d) means for applying an adjustable electric field to the Bragg grating
reflector to change the refractive index of the electro-optic material to
change the frequency of radiation produced by the frequency tunable
waveguide extended cavity laser so as to be at a desired frequency.
In an alternate version of the invention the frequency tunable waveguide
extended cavity laser is used as a tunable narrow-bandwidth pump source
for a waveguide second harmonic optical frequency converting portion which
is integrated into the same channel waveguide containing the frequency
tunable Bragg grating reflector. A QPM waveguide portion is preferably
chosen as the nonlinear optical frequency converting portion because of
its usefulness in converting infrared radiation to visible light, high
performance (as recognized in the art), and compatibility with a frequency
tunable Bragg grating reflector.
In yet another version of the invention, additional benefits are realized
by placing the waveguide nonlinear optical frequency converting portion
between the laser diode and the Bragg grating reflector. In this extended
cavity scheme the waveguide nonlinear optical frequency converting portion
takes advantage of intracavity optical power enhancement increasing the
efficiency of the nonlinear optical frequency conversion process.
ADVANTAGES
Frequency tunable waveguide extended cavity lasers according to this
invention provide a compact, tunable, narrow linewidth, coherent radiation
source.
The invention maintains optimum efficiency in nonlinear optical frequency
conversion schemes such as the second harmonic generation of light via
quasiphase matching as a result of drifts in temperature, wavelength, or
to compensate for device fabrication errors. The invention also increases
the efficiency of the generated second harmonic power via intracavity
enhancement of the fundamental. This permits the use of lower power and
hence lower cost laser diode pump sources.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a, 1b and 1c show top, side and front views of a frequency tunable
waveguide extended cavity laser in accordance with this invention;
FIGS. 2a and 2b show an alternative embodiment (top and front views) of a
frequency tunable waveguide extended cavity laser in accordance with this
invention. The electrodes are positioned on a z-cut LiNbO.sub.3 wafer as
to produce an electric field parallel to the c-axis as shown;
FIGS. 3a and 3b show an alternative embodiment (top and front views) of a
frequency tunable waveguide extended cavity laser in accordance with this
invention. The electrodes are positioned on a x-cut or y-cut LiNbO.sub.3
wafer as to produce an electric field parallel to the c-axis as shown;
FIGS. 4a, 5b and 4c show a frequency tunable waveguide extended cavity
laser. In this embodiment a waveguide frequency converting portion is
situated outside the cavity of the frequency tunable waveguide extended
cavity laser; and
FIGS. 5a and 5b show a frequency tunable waveguide extended cavity laser.
In this embodiment a waveguide frequency converting portion is situated
inside the cavity of the frequency tunable waveguide extended cavity laser
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
LiNbO.sub.3, LiTaO.sub.3, and KTiOPO.sub.4 are preferred nonlinear optical
materials for electro-optic and waveguide QPM devices. The following
description refers to an embodiment of the invention which employs a
commercially available Fabry-Perot laser diode which is coupled to a
LiNbO.sub.3 waveguide. The invention could as well be applied to other
nonlinear optical materials. A frequency tunable waveguide extended cavity
laser is shown schematically in FIGS. 1a, 1b and 1c. The laser diode 1 has
an anti-reflective (AR) front facet coating 2 instead of the usual
partially reflective coating. The AR coating is preferred although it is
recognized that the device will still operate with a partially reflective
coating. The reflecting rear facet coating 3 is conventional. This facet
forms one end of the frequency tunable waveguide extended cavity laser.
The laser diode 1 is end-coupled to a LiNbO.sub.3 channel waveguide 4
which is fabricated from a polished single crystal LiNbO.sub.3 wafer 5.
Other coupling techniques such as those described in U.S. Pat. No.
5,185,752 to Welch et al could also be used. The embodiment employs a
conventional surface modified LiNbO.sub. 3 channel waveguide which is
produced by a proton-exchanged process. Such waveguide production
techniques are well known in the art. It is recognized that other
waveguide types could be used such as waveguides made from transparent
thin film nonlinear organic or nonlinear inorganic materials. Examples of
these alternate waveguide types are well known in the art. It is preferred
that the channel waveguide have lateral dimensions such that the amount of
radiation coupled from the laser into the channel waveguide is maximized
while still supporting only the lowest order mode. If a z-cut LiNbO.sub.3
wafer incorporating a conventional proton-exchanged waveguide is used then
the waveguide will support only TM modes. (See D. F. Clark, A. C. G. Nutt,
K. K. Wong, P. J. R. Laybourn, and R. M. De La Rue, J. Appl. Phys. 54 6218
1983). TM mode support occurs because the index of refraction is higher in
the proton-exchanged region (as compared to the surrounding region) in a
direction along the c-axis only, i.e., in a direction perpendicular to the
plane of the wafer. In the direction parallel to the plane of the wafer
the index of refraction is lower in the proton-exchanged region as
compared to the surrounding region. Therefore, only for radiation
polarized parallel to the c-axis can waveguiding occur. For the case of
x-cut or y-cut LiNbO.sub.3 wafers incorporating a conventional
proton-exchanged channel waveguide only transverse electric (TE) modes
would be supported. (See M. M. Abouelleil and F. J. Leonberger, J. Am
Ceram. Soc. 72, 1311 1989). Although polarization selection may be
achieved by rotation of the waveguide relative to the laser diode, or
through the use of a half-wave plate, neither of these techniques are
compatible with a low cost planarization technology. It is recognized that
other electro-optic based polarization selection techniques that are
compatible with planarization technology may be used. The waveguide AR
coatings are placed by standard deposition techniques on the input facet
6a and output facet 6b of the LiNbO.sub.3 channel waveguide 4. For
example, a quarterwave layer of SiO.sub.2 evaporated on the polished
waveguide input and output facets should be sufficient.
A Bragg grating reflector 7 is incorporated into the LiNbO.sub.3 channel
waveguide 4 completing the cavity of the frequency tunable waveguide
extended cavity laser. This Bragg grating reflector 7 can be produced by
first forming a periodic masking layer with photoresist exposed by
standard holographic techniques and then using standard ion-milling to
remove material in the unmasked regions. An alternative method for
producing a Bragg grating reflector involves the deposition of a thin film
layer of an optically transparent material on the surface of the
LiNbO.sub.3 wafer 5 where then a periodic masking layer is formed and
etched as described above. The thin film material may or may not be a
nonlinear optical material. It may be organic or inorganic. It should have
an index or refraction that is close to but not significantly higher than
the LiNbO.sub.3 channel waveguide index of refraction. In addition, it
should not have an index of refraction lower than the buffer layer 10
index of refraction. Examples of qualifying materials include Ta.sub.2
O.sub.5 and Si.sub.3 N.sub.4 (assuming a buffer layer of SiO.sub.2). The
LiNbO.sub.3 channel waveguide 4 has a length extending along the entire
LiNbO.sub.3 wafer. The LiNbO.sub.3 wafer is long enough to accommodate the
Bragg grating reflector 7. It may be necessary in the TM polarization case
for an additional length of LiNbO.sub.3 channel waveguide between the
input facet 6a and the Bragg grating reflector 7 to extend beyond the
evanescent TE mode decay length to ensure lasing of the inherently lower
gain TM mode. The distance between the Bragg grating reflector 7 and the
output facet 6b may have zero length or may be longer depending on the
application. The two most important optical properties of the Bragg
grating reflector 7 which impact device operation and performance are the
reflectivity and bandwidth.
It is desirable to produce a Bragg grating reflector 7 such that the
reflectivity is commensurate with optimum output coupling for the laser
diode 1. Typically, this value is in the range of a few percent. The exact
value for reflectivity will depend on the details of the laser diode as
well as the coupling efficiency of the laser diode 1 to the LiNbO.sub.3
channel waveguide 4. In general, it is desirable to produce a Bragg
grating reflector with a small bandwidth. The exact value for this
bandwidth will depend on the application of the frequency tunable
waveguide extended cavity laser. The Bragg grating reflector 7
reflectivity increases with increasing length while the bandwidth
decreases. The Bragg grating reflector reflectivity and bandwidth are also
functions of the ratio of Bragg grating reflector depth to LiNbO.sub.3
channel waveguide depth (the reflectivity and bandwidth increase with an
increased ratio of Bragg grating reflector depth to LiNbO.sub.3 channel
waveguide depth) as well as the LiNbO.sub.3 channel waveguide effective
index of refraction (as the LiNbO.sub.3 channel waveguide effective index
of refraction decreases, the reflectivity and bandwidth increase). The
Bragg grating reflector length may usefully range between 0.05 mm and 20
mm depending on the values of the other parameters. Preferred values would
range from 0.5 mm to 10 mm. Bragg grating reflector depths usefully range
from 50 .ANG. to the depth of the LiNbO.sub.3 channel waveguide mode.
There is no advantage in a Bragg grating reflector depth extending beyond
the LiNbO.sub.3 channel waveguide mode depth. LiNbO.sub.3 channel
waveguide mode depths typically range from 1 to 2 .mu.m in a proton
exchanged waveguide where the effective index of refraction typically
ranges from 0.01 to 0.15 below the (LiNbO.sub.3) substrate index of
refraction. The exact values would depend on the details of the
fabrication process. The period of the grating A (see FIG. 1) is given by
the Bragg condition
.LAMBDA.=m(.lambda./2N.sub.eff),
where .lambda. is the wavelength at peak reflectivity, N.sub.eff is the
waveguide effective index of refraction, and m is the grating order. It is
desirable, in many cases, to use as low a grating order as possible in
order to produce a grating with a small bandwidth (for a given length).
The period for a first order grating at a wavelength of .lambda.=860 nm
made from a typical LiNbO.sub.3 proton-exchanged channel waveguide would
be approximately 0.2 .mu.m while a third order grating would be
approximately 0.6 .mu.m.
FIGS. 1a, 1b and 1c illustrate top 8 and bottom 9 metal electrodes placed
such that an applied voltage results in an electric field oriented
perpendicular to the plane of the wafer. In the case of the z-cut
LiNbO.sub.3 wafer (TM polarization) this electrode orientation would allow
use of the large electro-optic coefficient r.sub.33. The top 8 and bottom
9 electrodes have dimensions which correspond at least to the dimensions
of the intersection of the LiNbO.sub.3 channel waveguide 4 portion and the
Bragg grating reflector 7. Somewhat larger electrode dimensions are
desirable in order to assure uniform electric fields in said intersecting
area. To avoid unnecessary light loss in the LiNbO.sub.3 channel waveguide
portion which lies between the electrodes, it is preferable to include a
buffer layer 10 between the top electrode 8 and the Bragg grating
reflector 7. The buffer layer 10 should be transparent at both the
fundamental and the second harmonic frequencies. It may be electrically
insulating or conducting. SiO.sub.2 is an example of a preferred inorganic
insulating buffer layer material. Organic materials are also useful as
buffer layers. The buffer layer 10 thickness should be at least such that
the evanescent field of the guided mode does not extend to the conducting
top metal electrode 8. The specific refractive index of the buffer layer
10 will influence the LiNbO.sub.3 channel waveguide modes and cut-off
thicknesses and so must be taken into account in the design. In the
present specific embodiment it can be assumed for simplicity that the
buffer layer 10 extends over the entire LiNbO.sub.3 wafer 5 surface.
The thickness of the LiNbO.sub.3 wafer 5 is determined by a tradeoff
between required mechanical rigidity and control voltage level, since the
electro-optic control function is an electric field sensitive quantity.
Minimum LiNbO.sub.3 wafer thickness is about 50 .mu.m. It is possible to
gain advantage by employing a LiNbO.sub.3 wafer thickness capable of
providing excellent mechanical properties and circumventing the problem of
increased voltage requirement by milling the LiNbO.sub.3 wafer in way of
the bottom metal electrode portion to a smaller LiNbO.sub.3 wafer
thickness thus reducing the required voltage level.
It is recognized that z-cut LiNbO.sub.3 wafer surface electrodes 11, 12 and
13 illustrated in FIGS. 2a and 2b along with the voltage polarities shown
will result in a component of the electric field directed along the c-axis
permitting voltage controlled frequency tuning of the laser diode via the
electro-optic coefficient r.sub.33. The voltage is applied and adjusted
via a voltage adjusting system 14. Such a system is well known in the art.
In this configuration a buffer layer 10 is still necessary, however, the
LiNbO.sub.3 wafer need not be thinned.
For x-cut or y-cut LiNbO.sub.3 wafers (TE polarization) it is preferred
that the applied electric field be in a direction parallel to the plane of
the wafer so that the electro-optic coefficient r.sub.33 can be used. It
is recognized that the surface electrode geometry illustrated in FIGS. 3a
and 3b would suffice. x-cut or y-cut LiNbO.sub.3 wafer surface electrodes
16 and 17 placed on either side of the LiNbO.sub.3 channel waveguide 4
would result in the appropriate field direction. A voltage adjusting
system 14 would permit frequency tuning of the laser diode. In this case a
buffer layer would not be necessary nor would the wafer need to be
thinned.
As stated previously, optical feedback provided to the laser diode by the
Bragg grating reflector will cause the laser diode to oscillate at the
wavelength selected by the Bragg grating reflector. The wavelength,
.lambda. at which the Bragg grating reflector reflects the most
efficiently is given by the Bragg condition stated previously:
.lambda.=2n(.LAMBDA./m).
In this last expression it is assumed for simplicity that for a proton
exchanged LiNbO.sub.3 channel waveguide N.sub.eff .apprxeq.n, where n is
the LiNbO.sub.3 wafer index of refraction. This expression illustrates the
ability to change the wavelength at which the Bragg grating reflector
reflects the most efficiently and hence when coupled to a laser diode the
wavelength at which the laser diode will oscillate by changing the index
of refraction. The change in lasing wavelength, .DELTA..lambda. is related
to the index change, .DELTA.n by
.DELTA..lambda.=.lambda.(.DELTA.n/n).
From the previously described geometries, the index of refraction can be
changed through the electro-optic coefficient, r.sub.33 by application of
an electric field, E as given by
.DELTA.n(E)=-n.sup.3 r.sub.33 E/2.
For the TM polarization case and the electrode geometry of FIG. 1, the
electric field, E is directly proportional to the applied voltage V, the
constant of proportionality being related to the electrode separation,
buffer layer thickness and dielectric constants of the electro-optic and
buffer layer materials. In the typical occurrence where the buffer layer
thickness is much smaller than the electrode separation, and their
dielectric constants are of the same order, then the electric field, E is
given approximately by
E=V/d,
where d is the electrode separation. Combining the last three expressions
yields
.DELTA..lambda.=-(.lambda. n.sub.e.sup.3 r.sub.33 /2)(V/d),
where the extraordinary index of refraction, n.sub.e is used.
An input wavelength of 860 nm is used. Such a wavelength may be
conveniently produced by commercial diode lasers. From the published
literature (See S. Fries and S. Bauschulte, Phys. Stat. Sol. (a) 125, 369
1991) it can be found with some interpolation, that for LiNbO.sub.3,
n.sub.3 (860 nm)=2.168 and
r.sub.33 (860 nm)=29.9.times.10.sup.-12 m/V.
Thus the change in the lasing wavelength is given by
.DELTA..lambda.=-(6.043.times.10.sup.-2)(V/d)
in units of .ANG. when V is in kV and d is in cm. This implies a field of
16.5 kV/cm is needed per .ANG. of tuning. Fields providing a few .ANG. to
1 nm of tuning should be realizable with reasonable voltages by using the
electrode geometries discussed above.
In another embodiment of the invention the tunable laser diode device is
integrated with a periodicallyopoled QPM waveguide portion to produce
second harmonic light. It is recognized that other phase matching
techniques could be used such as those based on birefringence or waveguide
optical mode matching. Such techniques are well known in the art. Such a
device would be useful to convert infrared radiation to visible light.
This device is shown schematically in FIGS. 4a, 4b and 4c. The device
contains the same elements as the device in FIGS. 1a, 1b and 1c with the
addition of a periodically-poled QPM portion 19 which is fabricated on the
same substrate as the Bragg grating reflector 7. In this case the channel
waveguide 4 (and LiNbO.sub.3 substrate 5) is extended along the entire
periodically poled QPM portion 19. The period of the Bragg grating
reflector 7 is designed to match (within the tuning range of the Bragg
grating reflector) the input fundamental wavelength of maximum frequency
conversion of the periodically-poled QPM portion 19. For maximal second
harmonic power conversion the TM polarized version of the device is used.
This would allow use of the large r.sub.33 electro-optic coefficient for
grating tuning described earlier while also using the large nonlinear
optical coefficient, d.sub.33 for second harmonic conversion. The
dimension of the periodically poled QPM portion may be usefully between 1
mm and 30 mm in length, and preferably between 3 mm and 20 mm with the
optimal length being between 5 mm and 15 min. The LiNbO.sub.3 channel
waveguide portion extending beyond the periodically poled QPM portion may
have zero length or may extend on both sides of the periodically poled QPM
portion depending on the application. It is recognized that surface
electrodes 11, 12, and 13 of the type illustrated in FIGS. 2a and 2b may
be used instead of the electrodes 8 and 9 shown in FIGS. 4a, 4b and 4c.
The voltage applied to the Bragg grating reflector may be used to tune the
laser diode to maintain maximum second harmonic power. This optimization
may be done manually or through an electronic means via a feedback loop 25
which monitors the second harmonic power with a photodiode 26 and adjusts
the laser frequency accordingly by application of a voltage to the grating
electrodes via a feedback controlled voltage adjusting system 27. Such
control systems are well known in the art.
An alternate embodiment of this device illustrated in FIGS. 5a and 5b
places the periodically poled QPM portion 19 at the input end of the wafer
in between the laser diode 1 and the Bragg grating reflector 7. In this
case the Bragg grating reflector should be made highly reflecting at the
fundamental wavelength while transmissive at the second harmonic
wavelength. The Bragg grating reflector can be made transmissive at the
second harmonic wavelength due to the inherent LiNbO.sub.3 substrate
material dispersion. For example, a first order grating designed to be
highly reflective at a fundamental wavelength of .lambda.=860 nm in
LiNbO.sub.3 (using a value of n.sub.e =2.168) would need a period of 0.2
.mu.m. Because of the material dispersion the Bragg grating reflectivity
peak wavelength nearest to the second harmonic wavelength (.lambda.=430
nm) would approximately occur at .lambda.=460 nm. As long as this
wavelength was outside the reflectivity band of the Bragg grating
reflector, the second harmonic will pass unattenuated. The device in this
configuration is designed to take advantage of intracavity power
enhancement at the fundamental wavelength. (See Zernike and Midwinter,
Applied Nonlinear Optics [Wiley, New York 1973] Chap. 5). The higher
circulating power within the cavity (the cavity is defined at one end by
the laser diode rear facet coating 3 and on the other end by the Bragg
grating reflector 7) at the fundamental wavelength would produce a larger
amount of second harmonic power. The exact power enhancement will depend
on the net gain of the cavity relative to the net loss. The gain in the
cavity is provided by the laser diode while the loss is incurred through a
number of items including the laser diode 1 to LiNbO.sub.3 channel
waveguide 4 coupling loss; losses in the LiNbO.sub.3 channel waveguide
from scattering and absorption; reflectivity losses from the laser diode
rear coating 3 and the Bragg grating reflector 7; and losses to the
fundamental from the second harmonic conversion process. It is recognized
that tuning of the laser diode via a voltage applied to the Bragg grating
reflector for optimization of the generated second harmonic power may be
accomplished by a similar system as outlined in the preceding paragraph.
The invention has been described in detail with particular reference to
preferred embodiments thereof, but it will be understood that variations
and modifications can be affected within the spirit and scope of the
invention.
PARTS LIST
1 laser diode
2 front facet coating of laser diode
3 rear facet coating of laser diode
4 LiNbO.sub.3 channel waveguide
5 LiNbO.sub.3 wafer
6a waveguide input facet
6b waveguide output facet
7 Bragg grating reflector
8 top metal electrode
9 bottom metal electrode
10 buffer layer
11 z-cut LiNbO.sub.3 surface electrode
12 z-cut LiNbO.sub.3 surface electrode
13 z-cut LiNbO.sub.3 surface electrode
14 voltage adjusting system
16 x-cut or y-cut LiNbO.sub.3 surface electrode
17 x-cut or y-cut LiNbO.sub.3 surface electrode
19 periodically poled QPM portion
25 feedback loop
26 photodiode
27 feedback controlled voltage adjusting system
* * * * *
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