JPH02125482A - Laser frequency stabilizer - Google Patents

Abstract

PURPOSE:To provide high frequency stability by injecting all or part of harmonics to an injection synchronous oscillator to generate a light synchronized in phase, inputting it to a light frequency reference, and stabilizing the frequency of a laser. CONSTITUTION:Part of a light radiated from a long wavelength band semiconductor laser 1 (1.56mum of wavelength) is converted to harmonic (0.78mum of wavelength) by a second harmonic wave generator 7 using a nonlinear optical crystal such as KDP, etc. This harmonic wave is injected to a short wavelength band semiconductor laser 9. Part of the output is incident to rubidium cell 8 to saturate the absorption of the other of the output of the laser 9. When the injected current of the laser 1 is modulated by an optical modulation oscillator 5, the absorbed light of the cell 8 is modulated. The frequency of the laser 1 is controlled by an injecting current controller 6 on the basis of the output synchronously detected by a synchronous detector 4, the frequency of the laser 1 can be stabilized to double of the resonance frequency of the rubidium atom.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser frequency stabilizing device for realizing a laser light source with high frequency stability.

[Problems to be solved by conventional technology/inventions] In recent years, as the spectral purity and frequency variation of semiconductor lasers have improved, research and development of light wave communication systems and various coherent optical fiber measurement devices have been actively conducted. There is. When constructing these, a laser light source serving as a frequency reference having high frequency stability in a long wavelength band (1.3 to 1.6 ui) is required.

FIG. 2 shows the configuration of a laser frequency stabilizing device using a conventional long wavelength band semiconductor laser. Long wavelength semiconductor laser
A part of the light emitted from the cell enters the molecular cell 2. - If the frequency of the long wavelength band semiconductor laser l matches the resonance frequency of the molecules sealed in the molecular cell 2, the light incident on the molecular cell 2 will be absorbed. Here, when the injection current of the long wavelength band semiconductor laser 1 is modulated using the optical modulation oscillator 5, the frequency of the long wavelength band semiconductor laser 1 is modulated, and therefore the amount of light absorbed by the molecular cell 2 is also modulated.

The transmitted light of the molecular cell 2 is converted into an electrical signal by a photodetector 3, and then synchronously detected by a synchronous detector 4 using the output of an optical modulation oscillator 5 as a reference signal.

At this time, the output of the synchronous detector 4 is the output of the long wavelength band semiconductor laser 1.
It is proportional to the difference between the frequency of and the resonance frequency of the molecules sealed in the molecular cell 2. Based on the output of the synchronous detector 4, the injection current controller 6 controls the frequency of the long-wavelength semiconductor laser l, thereby changing the frequency of the long-wavelength semiconductor laser l to the frequency of the molecules sealed in the molecular cell 2. It can be stabilized to a resonant frequency.

Generally, the narrower the linewidth of the absorption spectrum of a molecule or atom used as an optical frequency standard, the greater the frequency discrimination sensitivity of the optical frequency standard and the better the frequency stability. Furthermore, since the value of the transition probability of a molecule or atom is proportional to the product of the oscillator strength of that molecule or atom and the incident photon flux, if a molecule or atom with a large oscillator strength is used,
The amount of absorbed light increases, the SN ratio of the resonance signal increases, and the frequency stability improves.

Ammonia molecules or the like are usually used as the absorption medium in the long wavelength band. However, since the oscillator strength of ammonia molecules and the like is not very large and the absorption spectrum has a Doppler spread, very high frequency stability cannot be obtained.

On the other hand, alkali metal atoms such as rubidium and cesium have high oscillator strength, and if methods such as saturated absorption and atomic beams are used, an absorption spectrum without Doppler broadening can be obtained. However, since rubidium has an absorption line near 0.78μ and cesium has an absorption line near 0.85itx, they cannot be used as they are for a laser frequency stabilization device in a long wavelength band.

Therefore, a laser frequency stabilizing device as shown in FIG. 3 can be considered. Long wavelength band semiconductor laser 1 (wavelength 1.56
A part of the light emitted from the wavelength of
8μ, w). This harmonic is the rubidium cell 8
is incident on the If the frequency of the harmonics matches the resonance frequency of rubidium atoms, the harmonics incident on the rubidium cell 8 will be absorbed. Here, when the injection current of the long wavelength band semiconductor laser I is modulated using the optical modulation oscillator 5, the frequency of the long wavelength band semiconductor laser 1 is modulated, so the amount of light absorbed by the rubidium cell 8 is also modulated.

The transmitted light of the rubidium cell 8 is converted into an electric signal by the photodetector 3, and then synchronously detected by the synchronous detector 4 using the output of the optical modulation oscillator 5 as a reference signal. At this time, the synchronous detector 4
The output of is proportional to the difference between the frequency of the long wavelength band semiconductor laser l and the resonance frequency of rubidium atoms divided by 2. Based on the output of the synchronous detector 4, the injection current controller 6 controls the frequency of the long wavelength semiconductor laser l, thereby stabilizing the frequency of the long wavelength semiconductor laser l to twice the resonance frequency of rubidium atoms. can be converted into

However, since current nonlinear optical crystals have low nonlinear susceptibility, their conversion efficiency to harmonics is also very low. Therefore, although the oscillator strength of rubidium atoms is large, the incident photon flux is small, so the S/N ratio of the resonance signal is also small, and very high frequency stability cannot be obtained.

The present invention aims to solve the above-mentioned problems in conventional laser frequency stabilization devices using nonlinear optical crystals and alkali metal atoms, and to realize a laser frequency stabilization device that has high frequency stability in the 1.5 ux band. shall be.

[Means to solve the problem]

The present invention provides a laser, an optical harmonic generation means for generating harmonics from the output light of the laser, an injection-locked oscillator for generating light phase-locked with the output of the optical harmonic generation means, and an optical frequency standard. , a laser frequency control means, a part of the output light of the laser is incident on the optical harmonic generation means to generate harmonics, and all or a part of the harmonics are sent to the injection-locked oscillator. generating light that is phase-locked to the harmonic, inputting all or part of the light that is phase-locked to the harmonic to the optical frequency reference, and controlling the laser frequency based on the output of the optical frequency reference. The invention is characterized in that the frequency of the laser is stabilized by means.

In the present invention, a short wavelength semiconductor laser is used as an injection-locked oscillator, and the second harmonic of a long wavelength semiconductor laser is
After faithfully amplifying the phase, frequency stabilization is performed using, for example, an alkali metal vapor cell. Since the intensity of the light incident on the cell is sufficiently high, the S/N ratio when detecting the resonance signal becomes very large.

Furthermore, since it is also possible to use the saturation absorption method, very high frequency stability can be obtained.

〔Example〕

FIG. 1 shows an example of the configuration of the laser frequency stabilizing device described in the claims. Long wavelength band semiconductor laser 1 (wavelength 1.56
A part of the light emitted from the μ shoulder (laser in the claims) is converted into harmonics (
wavelength 0.78 μR). This harmonic is injected into a short wavelength band semiconductor laser 9 (wavelength: 0.78 μm shoulder, injection-locked oscillator in the claims). If the frequency of the harmonic and the free-running frequency of the short wavelength semiconductor laser 9 are sufficiently close, the short wavelength semiconductor laser 9 oscillates in phase synchronization with the harmonic.

The frequency of this short wavelength band semiconductor laser 9 is the same as the frequency of the harmonic, and its intensity is much greater than that of the harmonic.

A part of the output of the short wavelength semiconductor laser 9 (saturated light) is incident on the rubidium cell 8 (optical frequency reference in the claims) to absorb the other output of the short wavelength semiconductor laser 9 (probe light). saturate. That is, if the frequency of the short wavelength band semiconductor laser is sufficiently close to the resonance frequency of rubidium atoms (about the natural width), the amount of light absorbed by the probe light is reduced. Here, when the injection current of the long wavelength band semiconductor laser 1 is modulated using the optical modulation oscillator 5, the frequency of the long wavelength band semiconductor laser 1 is modulated, so the amount of light absorbed by the rubidium cell 8 is also modulated. The probe light transmitted through the rubidium cell is converted into an electric signal by a photodetector 3, and then synchronously detected by a synchronous detector 4 using the output of an optical modulation oscillator 5 as a reference signal. At this time, the synchronous detector 4
The output of is proportional to the difference between the frequency of the long wavelength band semiconductor laser l and the resonance frequency of rubidium atoms divided by 2. Based on the output of the synchronous detector 4, the injection current controller 6 (laser frequency control means in the claims) controls the long wavelength band semiconductor laser l.
By controlling the frequency of the long-wavelength semiconductor laser l, it is possible to stabilize the frequency of the long-wavelength semiconductor laser l to twice the resonance frequency of the rubidium atom.

〔Effect of the invention〕

According to the present invention, since the intensity of the light incident on the optical frequency reference is sufficiently large, the S/N ratio during resonance signal detection is large and high frequency stability can be obtained. Therefore, the present invention can be used as an optical frequency standard when constructing a light wave communication system or a coherent optical fiber measurement device.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a laser frequency stabilizing device shown as an embodiment of the present invention, Fig. 2 is a block diagram of a conventional laser frequency stabilizing device using molecular cells, and Fig. 3 is a block diagram of a conventional laser frequency stabilizing device using a molecular cell. FIG. 1 is a block diagram of a conventional laser frequency stabilization device using rubidium cells. ■・・・Laser (long wavelength band semiconductor laser), 3.
. . . Photodetector, 4 . . . Synchronous detector, 5 . . . Optical modulation oscillator, 6 . . . Laser frequency control means (injection current controller) 7 ...... Optical harmonic generation means (second harmonic generator), 8... Optical frequency reference (ruvidium cell),
9... Injection-locked oscillator (short wavelength semiconductor laser) Applicant Nippon Telegraph and Telephone Corporation Agent Patent attorney Masatake Shiga

Claims (1)

[Scope of Claims] A laser, an optical harmonic generation means for generating harmonics from the output light of the laser, an injection-locked oscillator for generating light phase-locked with the output of the optical harmonic generation means, and an optical frequency. a reference, and a laser frequency control means, a part of the output light of the laser is incident on the optical harmonic generation means to generate harmonics, and all or a part of the harmonics are transferred to the injection locking means. injecting it into an oscillator to generate light that is phase-locked to the harmonic, inputting all or part of the light that is phase-locked to the harmonic to the optical frequency reference, and controlling the laser based on the output of the optical frequency reference. A laser frequency stabilizing device, characterized in that the frequency of the laser is stabilized by a frequency control means.