JPS60145681A - Laser device for stabilized mode synchronization - Google Patents

Abstract

PURPOSE:To enable the array of stable photo pulses to be obtained as the output by maintaining the state of mode synchronization conveniently and securely by a method wherein the spectrum width of an optical fiber emitted light is used as a control signal. CONSTITUTION:A control circuit 10 consists of an input circuit that converts the spectrum width of the photo reception output of a photo detector 9 into an array of electric pulses by A/D conversion, and of an operational ciruit. The operational circuit increases the frequency of a sine wave oscillator 11 by DELTAf; consequently, when the spectrum width detected by the photo detector 9, i.e. the number of pulses increase more than the number of pulses of a memory circuit, the former circuit increases the frequency by DELTAf also at the next cycle. When the frequency is increased further by DELTAf, resulting in the reduction in the number of pulses more than that of the memory circuit, the frequency is reduced by 2DELTAf at the next cycle. This control method enables the frequency to vary by + or -DELTAf with the optimum mode synchronization frequency as the center; however, an increase in DELTAf to approx. more than 10kHz exerts no effect on the mode syncronization laser output.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stabilized mode-locked laser device that can maintain a stable mode-locked state at all times.

Generally, in a solid-state laser or a dye laser, by mode-locking the oscillation longitudinal mode, an optical pulse train having a pulse width of subbyoseconds to subnanoseconds can be obtained. As the mode-locking method, a forced mode-locking method using an internal modulator is mainly used because the reproducibility of the generated optical pulse train is high.

In such a mode-locked laser device, the effective cavity length of the laser resonator changes due to heat generation in the laser medium, fluctuations in external temperature, etc., resulting in a detuning phenomenon of mode-locking and a stable mode. There was a problem that synchronized optical pulses could not be obtained for a long time (!: i, /1).As a countermeasure to this problem, the pulse width and emission timing of the output optical pulse could be detected using ultra-high-speed optical detection. It is possible to mechanically adjust the laser resonator length or finely adjust the modulation frequency of the internal modulator based on the changes in the output optical pulse detected by a sampling oscilloscope.However, such ultra-high speed The method using a photodetector requires a complicated control circuit, is accompanied by a great deal of trouble, and has the disadvantage that the device becomes bulky and unusable.

An object of the present invention is to eliminate such drawbacks and to provide a stabilized mode-locked laser device that is simple and capable of reliably maintaining a mode-locked state and producing a stable optical pulse train as an output.

The stabilized mode-locked laser device of the present invention includes a forced mode-locked laser oscillator in which a laser medium and a modulator are provided between two reflecting mirrors, and oscillation in which the frequency of the modulator is controlled by a predetermined control voltage. a frequency oscillator that provides an output;
A spectrum measuring device that measures the spectral width from the oscillation output light of the laser oscillator, and a control circuit that controls the control voltage of the frequency oscillator so that the spectral width signal output from the spectrum measuring device is maximized. configured.

The present invention utilizes the phenomenon that when detuning occurs in a forced mode-locked laser oscillator, the average output generally remains almost unchanged, but the pulse width of the output optical pulse increases and the peak power decreases.1. The present invention is characterized in that the self-phase modulation effect of the optical fiber is stabilized in the de-synchronized state by the change in the shape of the output optical pulse caused by the detuning.

Next, the self-phase modulation effect of an optical fiber will be briefly explained as the operating principle of the present invention. When a light pulse with a pulse width of sub-picoseconds to sub-nanoseconds is introduced into an optical fiber, a refractive index change occurs depending on the intensity of the light pulse, and there is a phenomenon in which the spectrum of this light pulse is broadened after exiting from the optical fiber. . This phenomenon is called the self-phase modulation effect, and the amount of spectrum broadening is large when the pulse width of the optical pulse is narrow and the peak power is high. Therefore, by maximizing the output spectrum width of the optical fiber, detuning can be minimized, that is, an optimal mode-locked state can be achieved.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a configuration diagram including an embodiment according to the present invention. In this figure, a forced mode-locked Lepe oscillator 20 includes a laser medium 1, reflectors 2 and 3, and an internal changer f for mode-locking.
Consisting of A device 4, 5 is an optical fiber, 6 is a lens, 7 is a reflecting mirror, 8 is a spectrum measuring device, 9 is a photodetector,
Reference numeral 10 indicates a control circuit for a mode-locked driver, and 11 indicates a mode-locked driver.

In this embodiment, the laser oscillator 20 uses a laser medium IjK
As No. 1, a Nd:YAG rod of 4 mm diameter x 75 mm was used, which was oscillated at a wavelength of 1.06 μm.

An ultrasonic modulator is used as the mode-locking internal modulator 4, and a mode-locking drino (11) is an 80 MHz sine wave oscillator (V) whose frequency can be controlled by voltage.
Mode synchronization is performed using CO). In addition, the optical fiber (as 5, core diameter 10 I'm s) y Ipa length IKyn, transmission loss 1 d at wavelength 1.06 μm
A single mode silica fiber of B/Km is used, and an odd diode (Ge-PD) is used as the spectrum measuring device 8.

The Fabry-Perot interferometer (8) is swept at 50 Hz, and the optical output after the sweep including information on the spectral width after being emitted from the optical fiber is received by a Ge-PDfq) and is controlled by an input circuit that converts it into an electric pulse train. , and an arithmetic circuit that calculates the required increase or decrease in the frequency of the sine wave oscillator, that is, the amount of detuning, according to the number of electric pulse trains, that is, the spectral width.

If the control procedure described below is called one cycle, this arithmetic circuit includes a memory circuit that stores the number of pulses from one cycle before, and determines whether to increase or decrease the frequency of the sine wave oscillator (11). It is decided as follows.

■) Is the frequency of the sine wave oscillator (11) Δff? As a result, if the spectral width detected by the photodetector 9, that is, the pulse number is equal to or increased by the number of pulses of the matching circuit, it is increased by Δf in the next cycle as well.

(2) If the frequency is further increased by Δf and as a result the number of pulses decreases below the number of pulses in the memory circuit, it is decreased by 2Δf in the next cycle.

According to this control method, the frequency changes by ±Δf around the optimal mode-locking frequency, but if Δf is 10 KHz
If the amount is set below, the mode-locked laser output will not be affected at all. In addition, in the mode-locked laser of this example, the maximum frequency increase or decrease required to compensate for detuning is a tooth height of 50 KHz, and the ultrasonic modulator (
4) Stabilization of the mode-locked state can be achieved in stages over a long period of time without changing the modulation depth. Note that this configuration cannot track detuning phenomena faster than 50 Hz, which is the sweep frequency of the Fabry-Perot interferometer, but mode-locking detuning phenomena are mainly caused by heat generation in the laser medium, fluctuations in external temperature, etc. This is gradual in terms of time and can be sufficiently stabilized with this configuration.

As explained above, according to the present invention, by using the spectral width of the light emitted from an optical fiber as a control signal, there is no need for a conventional ultra-high-speed photodetector and a high-speed and complicated control circuit, and the configuration can be simplified. This has the advantage that a simple and inexpensive device can be obtained.

Note that the present invention is not limited to the configuration of the embodiment, and several modifications can be made. For example, as the spectrum analyzer 8, a spectrometer or a prism, and as the optical fiber 5,
An optical fiber having a core of Gem, P, or O may be used. Furthermore, the modulation output power of the mode-locked driver 11 and the positions of the reflecting mirrors 2 and 3 may be controlled by the control circuit 10. Furthermore, the forced mode-locked laser oscillator 2 is equipped with an Ar laser pumped dye laser and a YAQ
Of course, it can also be used with other lasers of the same excitation type, such as an L/-excitation color center laser.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment according to the present invention. In the figure, 1...Laser medium, 2, 3.7 River...Reflection pleasure, 4...Internal modulator for mode locking, 5...
...Optical fiber, 6... Lens, 8...
・Spectrum measuring device, 9...Photodetector, 10・
... control circuit, 11 ... mode-locked driver, 20 ... forced mode-locked laser oscillator. Swamp 1

Claims (1)

[Claims] a forced mode-locked laser oscillator in which a laser medium and a modulator are provided between two reflecting mirrors; a frequency oscillator that supplies an oscillation output whose frequency is controlled by a predetermined control voltage to the modulator; and the laser oscillator. A stabilization mode that includes a spectrum measuring device that measures the spectral width from the oscillation output light of the oscillator, and a control circuit that controls the control voltage of the frequency oscillator so that the spectral width signal output from the spectrum measuring device is maximized. Synchronous laser equipment.