Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-05-22
2004-05-04
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S072000
Reexamination Certificate
active
06731660
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method for tuning nonlinear frequency mixing devices to yield continuous frequency tuning ranges across degeneracy.
BACKGROUND OF THE INVENTION
Light sources are the heart of most modern optics systems. Specifically, tunable light sources exhibiting a wide wavelength range and high output stability are the very foundation of telecommunications networks, optical testing equipment (e.g., swept wavelength testing systems) as well as laser processing tools. Many of these applications require coherent light sources with wide, stable and continuous tuning.
Nonlinear frequency mixing devices are often used to generate light in certain wavelength ranges where suitable laser sources are not available (e.g., due to lack of lasing media generating light in those wavelength ranges at sufficient power levels). Nonlinear frequency mixing is also used for optical signal processing of data-containing signals (e.g., wavelength conversion, chirp reversal, temporal multiplexing and de-multiplexing). Nonlinear optics encompass various processes by which a nonlinear optical material exhibiting a certain nonlinear susceptibility converts input light at an input wavelength to output light at an output wavelength. Some well-known nonlinear processes involving light at two or more wavelengths (e.g., three-wave mixing and four-wave mixing) include second harmonic and higher harmonic generation, difference frequency generation, sum frequency generation and optical parametric generation. The fundamentals of nonlinear optical processes are discussed extensively in literature and the reader is referred to Amnon Yariv,
Quantum Electronics
, 2
nd
edition, Wiley Press, 1967 for general information.
The prior art teaches the use of nonlinear frequency mixers in signal processing. For example, M. H. Chou et al., “1.5-&mgr;m-band wavelength conversion based on difference-frequency generation in LiNbO
3
waveguides with integrated coupling structures”, Optics Letters, Vol. 23, No. 13, Jul. 1, 1998 teach optical frequency mixing in the 1.5 &mgr;m wavelength band for telecommunication purposes. Additional information related to nonlinear wavelength conversion for communications applications can be found in I. Brenner et al., “Cascaded &khgr;
(2)
wavelength converter in LiNbO
3
waveguides with counter-propagating beams”, Electronics Letters, Vol. 35, No. 14, Jul. 8, 1999; and M. H. Chou et al., “Stability and bandwidth enhancement of difference frequency generation (DFG)-based wavelength conversion by pump detuning”, Electronics Letters, Vol. 36., No. 12, Jun. 10, 1999. Though these devices were tunable, none of them operated through degeneracy.
The output light from nonlinear wavelength converters can be tuned over a certain wavelength range. In general, control of the wavelengths of the mixing or interacting input light beams can be used to adlust the output wavelength. When the nonlinear conversion process takes place in materials specially engineered to achieve high nonlinear conversion efficiencies, e.g., materials using quasi-phase-matching (QPM) gratings in in-diffused waveguides, control over certain grating parameters (i.e., the phasematching condition) can be employed to achieve output wavelength tuning. For general information on this subject the reader is referred to Michael L. Bortz's Doctoral Dissertation entitled “Quasi-Phasematched Optical Frequency Conversion in Lithium Niobate Waveguides”, Stanford University, 1995 as well as M. L. Bortz et al., “Increased Acceptance Bandwidth for Quasiphasematched Second Harmonic Generation in LiNbO
3
Waveguides”, Electronics Letters, Vol. 30, Jan. 6, 1994, pp. 34-5. Additional information on devices using QPM gratings for nonlinear conversion is found in U.S. Pat. No. 5,875,053. The processes used to make QPM gratings are described in U.S. Pat. No's. 5,800,767 and 6,013,221, and waveguides with QPM gratings employed for nonlinear optical processes are described in U.S. Pat. No. 5,838,720. Some specific high power pumped mid-IR wavelength systems using non-linear frequency mixing to obtain tunable light sources are taught by Sander et al, in U.S. Pat. No. 5,912,910.
The prior art also teaches the use of nonlinear frequency mixing in light sources. The use of optical parametric oscillators as tunable light sources is discussed by Mark A. Arbore et al. in “Singly resonant optical parametric oscillation in periodically poled lithium niobate waveguides”, Optics Letters, Vol. 22, No. 3, Feb. 1, 1997. Also, the use of optical parametric oscillation for producing a tunable, short pulse and high repetition rate light source is taught by Kent Burr et al., “High-repetition-rate femtosecond optical parametric oscillator based on periodically poled lithium niobate”, Applied Physics Letters, Vol. 70, 1997, pg. 3343. The tuning bandwidth for the idler beam in Burr's OPO extends from 1.68 &mgr;m to 2.72 &mgr;m and for the signal beam from 1.12 &mgr;m to 1.50 &mgr;m. However, the tuning of nonlinear conversion processes becomes problematic as one approaches degeneracy. Kent Burr et al. avoid degeneracy altogether in operating their OPO and hence do not generate any output in the wavelength range from 1.50 &mgr;m to 1.68 &mgr;m. In other words, they do not provide a light source with a continuous tuning range.
To better explain the problem of degenerate operation we will initially review a typical optical parametric oscillator (OPO)
1
, as shown in FIG.
1
. OPO
1
has a tunable laser source
2
for providing a pump beam
3
at a pump frequency &ohgr;
p
. Pump beam
3
is in-coupled into a cavity
4
through an input coupler
5
. Cavity
4
contains an optical parametric amplifier (OPA)
6
which receives pump beam
3
and produces in response a signal beam
7
and an idler beam
8
. The output of OPO
1
is outcoupled from cavity
4
through output coupler
9
. The output of OPO
1
typically includes at least one of the generated beams, i.e., signal beam
7
and/or idler beam
8
. Optical parametric oscillation is supported by cavity
4
in OPA
6
and is a process during which pump beam
3
at pump frequency &ohgr;
p
transfers power to signal beam
7
at frequency &ohgr;
S
and to idler beam
8
at frequency &ohgr;
I
according to the equation:
&ohgr;
p
=&ohgr;
S
+&ohgr;
I
.
This process is performed such that energy and momentum are conserved between the photons of the three interacting beams, where in the case of quasi-phase-matching (QPM), momentum includes the k vector of the QPM grating. In the case where &ohgr;
S
=&ohgr;
I
=&ohgr;
p/2
the OPO is called degenerate and is essentially the opposite of second harmonic generation (SHG), such that:
&ohgr;
p
=2&ohgr;
p/2
.
In other words, degeneracy is encountered when frequency &ohgr;
S
of signal beam
7
and frequency &ohgr;
I
of idler beam
8
are equal to each other, and therefore equal to half of pump frequency &ohgr;
p
of pump beam
3
which is driving OPO
1
.
The approach to degeneracy and degeneracy itself are illustrated in
FIGS. 2A and B
. As seen in
FIG. 2A
, when pump beam
3
is tuned to a first pump frequency &ohgr;
p1
it establishes a gain spectrum A in OPA
6
with gain centered at a first signal frequency &ohgr;
S1
and at a first idler frequency &ohgr;
I1
. Consequently, signal beam
7
and idler beam
8
will experience gain within their respective gain regions of spectrum A and typically contain a range of frequencies within those gain regions. Since the gain regions of spectrum A are far apart (non-overlapping), first signal frequency &ohgr;
S1
does not at any point overlap with first idler frequency &ohgr;
I1
. It is therefore not possible for the same frequency to act as both signal and idler in this OPO.
As pump beam
3
is tuned to a second pump frequency &ohgr;p
2
, a gain spectrum B is produced with gain centered at a second signal frequency &ohgr;
S2
and at a second idler frequency &ohgr;
I2
. These two gain regions overlap a frequency range at
100
. If OPO cavity
4
Arbore Mark A.
Myers Lawrence E.
Ip Paul
Lightwave Electronics Corporation
Lumen Intellectual Property Services Inc.
Nguyen Tuan
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