Coherent light generators – Particular operating compensation means
Reexamination Certificate
1999-04-15
2001-09-04
Font, Frank G. (Department: 2877)
Coherent light generators
Particular operating compensation means
C372S033000, C372S022000, C372S026000, C372S018000, C372S019000
Reexamination Certificate
active
06285691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser light generating method and apparatus for generating laser light by locking the phases of a plurality of laser resonators to each other.
2. Description of the Related Art
The injection locking chain (refer to E. A. P. Cheng and T. J. Kane, Opt. Lett. 16, pp. 478-480 (1991)) is known as an example of laser light generating methods in which a plurality of laser resonators resonate simultaneously, that is, methods for generating laser light by locking the phases of a plurality of laser resonators to each other.
The injection locking chain will be described below with reference to FIG.
1
.
In the injection locking method, a small-size laser is used as a master laser
1
and laser light emitted from the master laser
1
is guided by a mirror
2
, an electro-optic modulator
3
, and a mirror
4
to a large-size slave laser (slave resonator)
22
that is composed of a precision positioning mirror
10
as a resonator optical path length control means and mirrors
5
,
7
, and
9
, whereby the laser light is amplified at a large gain without losing coherence.
In doing frequency locking in the slave resonator
22
, locking should be made to the frequency of the master laser light by precisely controlling the resonator optical path length of the slave resonator
22
. That is, the frequency in the resonator
22
is locked by a control loop that consists of a frequency supplying circuit (phase modulation signal oscillation circuit)
30
for supplying a phase modulation signal to the electro-optic modulator
3
, a photodetector
32
for detecting, as a phase detection signal, part of laser light exiting from the resonator
22
, and a mixer
31
for supplying a control signal (error signal or positioning signal) to the precision positioning mirror
10
by synchronously detecting the phase detection signal and the phase modulation signal.
As described above, the FM side band method is used as a frequency locking method. That is, the electro-optic modulator
3
as a phase modulation element is disposed in the optical path before the slave resonator
22
and frequency locking is performed by feeding back, as an error signal, a signal obtained by synchronously detecting the phase modulation signal for the electro-optic modulator
3
and return light from the slave resonator
22
to the precision positioning element
10
that is disposed in the resonator
22
.
The injection locking chain is a method for increasing the laser light output power by arranging, in series, a plurality of such slave resonators (slave resonators
22
and
23
) that operate according to the injection locking method. In this case, each slave resonator has an independent control loop according to the FM side band method and each electro-optic modulator is disposed on the optical path before the associated slave resonator. In the injection locking chain, it is necessary to cause the plurality of laser resonators to resonate simultaneously.
Laser light generating apparatuses that produce short-wavelength laser light by performing stepwise nonlinear wavelength conversion are known as other examples of laser light generating apparatuses in which a plurality of resonators resonate simultaneously (refer to A. Ashkin, G. D. Boyd, and J. M. Dziedzic, “Resonant optical second harmonic generation and mixing,” IEEE J. Quantum Electron. QE-2, pp. 109-124 (1966) ; W. J. Kozlovsky, C. D. Nabors, and R. L. Byer, “Efficient second-harmonic generation of a diode-laser-pumped CW Nd:YAG laser using monolithic MgO:LiNbO
3
external resonant cavities,” IEEE J. Quantum Electron. QE-24, pp. 913-919 (1988); and D. C. Gerstenberger, G. E. Tye and R. W. Wallace, “Efficient second-harmonic conversion of cw single-frequency Nd:YAG laser light by frequency locking to a monolithic ring frequency doubler,” Opt. Lett. 16, pp. 992-994 (1991)).
A method using an external resonator is known as a method for performing nonlinear wavelength conversion efficiently (refer to Z. Y. Ou, S. F. Pereira, E. S. Polzik, and H. J. Kimble, 17, pp. 640-642 (1992)). By using resonance of an external resonator, the nonlinear wavelength conversion efficiency can greatly be increased because of confinement of fundamental-wave laser light in the laser resonator. To cause the external resonator to resonate with fundamental-wave laser light, a precision positioning element in the resonator is controlled by using an FM side band method that is similar to the one used in the injection locking method.
A fourth harmonic wave with respect to fundamental-wave laser light can be generated by further wavelength-converting, by utilizing a nonlinear optical effect, a second harmonic wave generated in the above manner by SHG (Second Harmonic Generation). In this case, the wavelength shortening can be attained efficiently if the nonlinear wavelength conversion is performed by using an external resonator as in the case of the second harmonic generation process. Also in this short-wavelength laser light generation by the stepwise nonlinear wavelength conversion, it is necessary to cause a plurality of resonators to resonate simultaneously.
It goes without saying that it is necessary to cause a plurality of resonators to resonate simultaneously also in a laser light generating apparatus in which the above-described injection locking laser and nonlinear wavelength conversion process are combined.
As described above, in the laser light generating methods for generating laser light by causing a plurality of laser resonators to resonate simultaneously and establishing a phase-locked condition, control circuits are constructed independently for the control loops for controlling the resonator lengths of the laser resonators and the frequencies of phase modulation signals in all the control loops are set different from each other.
However, where phase modulation signals of the same frequency generated in a plurality of signal oscillation circuits are used simultaneously in different control loops, there may occur a case that a modulation signal in an upstream stage is mixed into an error signal in a downstream stage to disable a normal control. Further, where phase modulation signals generated by a single signal generation circuit are used in different control loops simultaneously, there may occur a case that an error signal cannot be generated correctly in a downstream loop. Still further, there may occur a case that beats of a plurality of non-lockable frequencies occur in accordance with the number of signal oscillators and hence undesired emission increases.
Usually, the Pound-Drever method (FM side band method) is used for high-precision control (locking) of a laser resonator. In the Pound-Drever method, an error signal having a bandwidth approximately equal to the control frequency bandwidth of an actuator for controlling the resonator length is generated from a phase signal of the resonator by double frequency down-conversion of “generation of a beat signal by a heterodyne method using a phase modulator (100 THz signal→100 MHz signal)” and “synchronous detection (10 MHz signal→1 kHz signal), ” whereby positional accuracy of about 100 THz (i.e., accuracy of the wavelength or lower) is secured. (The “generation of a beat signal by the heterodyne method” is also a kind of synchronous detection.)
Where a plurality of resonators are controlled by signals of the same frequency that are generated by different signal oscillators, there may occur a case that a signal caused by a phase variation in an upstream resonator is mixed into a signal in a downstream resonator to make it difficult to perform a high-precision control. Also where a plurality of resonators are controlled by phase signals generated by the same signal oscillator, there may occur a case that a temporal deviation between signal inputs to an upstream resonator and a downstream resonator causes a phase variation between the resonators to prevent correct generation of error signals as in the above example.
Further, in constructing an
Imai Yutaka
Kaneda Yushi
Kaneko Takeshi
Sakashita Tsutomu
Flores Ruiz Delma R.
Font Frank G.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sony Corporation
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