JPH03250680A - Frequency stabilized laser light source - Google Patents

Abstract

PURPOSE:To obtain a non-modulation high frequency stabilization optical output by controlling oscillation frequency of a first semiconductor laser to an absorption line frequency through current modulation, and further controlling oscillation frequency of a second semiconductor laser. CONSTITUTION:Output light of a semiconductor laser 1 controlled to an absorption line of a standard substance is emitted via a beam splitter 2 and combined through a polarized beam splitter 12 with output light of a semiconductor laser 10 emanating through a beam splitter 11. The combined light is matched in polarization plane at a polarized beam splitter 13 and thereafter a beat signal therebetween is detected by a photodetector 14. After the heat signal is mixed with an output of an oscillator 15, a mixed signal is subjected to synchronous detection in a synchronous detection circuit 17 taking an output from an oscillator 8 as a reference signal. A PI control circuit 18 controls oscillation frequency of a semiconductor laser 10 such that a synchronous detection output is 0. Thus, the oscillation frequency of the semiconductor laser 10 is shifted by the oscillation frequency of the oscillator 15 from the center frequency of the semiconductor laser 1 and is partly outputted via the beam splitter 11.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to improving the stability of a frequency-stabilized laser light source using a semiconductor laser.

<Prior art> Figure 7 is a block diagram showing a first conventional example of a frequency-stabilized laser light source, in which a semiconductor laser is directly modulated to control its oscillation frequency to the center of the absorption lines of atoms and molecules. It shows. The output light of the semiconductor laser 1 is separated into two directions by a beam splitter 2, and one of the lights enters an absorption cell 3 in which a standard substance is sealed. The light transmitted through the absorption cell 3 is detected by a photodetector 4, converted into an electrical signal, and input to a synchronous detection port #15 consisting of a lock-in amplifier or the like.

The oscillation frequency of the semiconductor laser 1 is current modulated by the output of the oscillator 8, and the synchronous detection circuit 5 performs synchronous detection using the output of the oscillator 8 as a reference signal. The current of the semiconductor laser 1 is controlled so that . The oscillation output of the PI control circuit 69 oscillator 8 and the output of the bias current source 9 are added by the adder circuit 7 and input to the semiconductor laser 1 . As a result, the semiconductor laser 1
The oscillation frequency of is controlled to the center of the absorption line of the atoms or molecules of the standard substance in the absorption cell 3, and the other output light of the beam splitter 2 has a frequency whose absolute value is determined by the atoms or molecules with high precision.

However, in the above-mentioned apparatus, since the output light is frequency-modulated, the instantaneous frequency is not stable, making it unsuitable for applications such as interferometric measurements, and has the drawback of narrowing the range of applications.

In order to solve these drawbacks, there is a structure in which a non-modulated output is obtained by externally modulating the output light of a semiconductor laser using an acousto-optic modulator and controlling it to atomic and molecular absorption lines.

<Problems to be Solved by the Invention> However, in the case of such a device, there is a problem that the stability is insufficient because the acousto-optic modulator is unstable.

The present invention has been made to solve these problems, and an object of the present invention is to realize a frequency-stabilized laser light source whose oscillation frequency is highly stably controlled with a simple configuration.

Means for Solving the Problems> The frequency stabilized laser light source according to the present invention includes a first semiconductor laser and an absorber encapsulating a substance that inputs the output light of the first semiconductor laser and absorbs it at a specific frequency. a first photodetector for detecting light transmitted through the absorption cell; a first oscillator for modulating the current of the first semiconductor laser; and an output of the first oscillator or an odd multiple thereof. a first synchronous detection circuit that inputs the output of the first photodetector using a frequency signal as a reference signal; a first control circuit that controls the absorption line of the absorption cell;
a second semiconductor laser; an optical means for multiplexing the output light of the second semiconductor laser; a second photodetector for detecting the beat signal of the output light output from the optical means; a second oscillator; and the output of the second
a second synchronous detection circuit that synchronously detects the output of the mixer using the output of the first oscillator or a signal of an odd multiple frequency thereof as a reference signal; Based on the output of the synchronous detection circuit, the first
a second control circuit that controls the oscillation frequency of the second semiconductor laser such that the difference between the oscillation frequency of the second semiconductor laser and the oscillation frequency of the second semiconductor laser is equal to the oscillation frequency of the second oscillator; It is characterized by being configured so that the output is unmodulated.

Furthermore, a third oscillator and an output of the second oscillator are connected to the third oscillator.
A modulation circuit that performs frequency modulation or phase modulation with the output of the third oscillator, and a mixer mixes the output of the modulation circuit and the output of the second photodetector, and outputs the output of the third oscillator or a signal of an odd multiple thereof. It may also be used as a reference signal for the second synchronous detection circuit.

Effect> The first semiconductor laser becomes highly stable through current modulation, and the oscillation frequency of the second semiconductor laser is controlled to be shifted from the center frequency of oscillation of the first semiconductor laser by the oscillation frequency of the second oscillator. Therefore, the frequency is unmodulated and highly stable.

<Example> The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram illustrating a first embodiment of a frequency left ventricular laser light source according to the present invention. 4 The same parts as in FIG. 6 are given the same symbols and their explanation will be omitted. 0 is a second semiconductor laser, 11 is a beam splitter that separates the output light of the semiconductor laser 10 into two directions, and 12 is an optical means that combines the light that has passed through the beam splitter 11 and the light that has passed through the beam splitter 2. Polarized beams constituting 1 liter, 1
3 is a polarizing beam splitter that aligns the plane of polarization so that the two lights interfere; 14 is a second photodetector that receives the light emitted from the polarizing beam splitter; 7 is a second photodetector that detects the beam hfS; The second oscillator, which performs a wave oscillator, 1
6 is a microwave mixer that mixes the output of the oscillator 15 and the output of the photodetector 14; 17 is a second lock-in amplifier that receives the output of the mixer 16 as an input 17 and uses the output of the oscillator 8 as a reference signal; 8 is a second P1 control circuit which inputs the output of the synchronous detection circuit 17 and applies a corresponding control output to the second semiconductor L-zer 10.

The operation of the apparatus having the configuration described above will be explained below. As described in the description of the conventional example, the output light of the semiconductor laser 1 controlled to have a standard IJ quality absorption line is emitted via the beam splitter 2, and is emitted via the beam splitter 11. ``The output light of the conductor sheaser 10 is combined by the polarization beam splitter 12.The light emitted from the polarization beam splitter 12 has its polarization plane combined by the polarization beam splitter 13, and then the light is combined by the photodetector 14. A signal is detected.;*v
After mixing this B-L-sign with the output of the oscillator 15 at x6, synchronous detection is performed in the synchronous detection circuit 17 using the output of the oscillator 8 as a reference signal. F) ■ The control circuit 18 has a synchronous detection output of O. As a result, the oscillation frequency of the semiconductor laser 10 is shifted from the center frequency of the semiconductor laser 1 to the oscillation frequency of the oscillator 15. is output.

This operation can be explained using a mathematical formula as follows. The electric field amplitude E1 of the semiconductor laser 1 is E = e 5inf
2rf1t −(Δf/fII)1 cos2rf L+F (t)) −(
1) Represented by 111. However, fl is the center frequency of the oscillation of the semiconductor laser, ψ1 is the phase noise of the semiconductor laser, f
is the oscillation frequency of the oscillator 8, and Δ is the maximum frequency deviation. Further, the electric field amplitude E2 of the semiconductor laser 0 is E = e sin (2yrf2t+si'2
(t) 't2... (2) Represented by: However, ψ2 (t) is the phase noise of the semiconductor laser 2. Photocurrent induced in photodetector 14 1.
. is 1 (X: (E + E2) PD 1 2e e cos (2π(f - f2) tl
2 1 +ψ(1,)-ψ2 is)-(Δf/f□)c.

S2πft + +I DC·= (3). Here, 91'(t)=P (t) -95,, ct>,
DC component I. If 0 is removed by AC coupling, I = K cos (2π(f, -f2)t, →
−ψPD 1 (t) −(Δ f/f )cos2 π
f □ Sat, )... (4) becomes. The output ■8 of the microwave oscillator 15 is V - =
e s i, n, 2 x f 3t
- (5) 3, the output ■HIx of Me8ga16 is V = K
[5in(2π(f −f −f3) old×
2 1 2 t5''ψ(t)−(Δf,'f)cos
2πf□t)-I sin (2r (fl-f
2+f3) t+so (t)(Af/f)co
s2πf, t, )]... (6) Now, considering only the components of f"fl"f2 f3, Vux-=K2 S], n! 2 yr f
t (Δf′fII)cos2πf t+p (t
) i =・(7). The spectral linewidth of a semiconductor laser has an F-Lentz distribution, and if the half-width of the spectral line is Δf1, then (7
) has a Lorentz distribution, and the l-signal has a frequency f□
, it is thought to vary sinusoidally with a maximum frequency shift Δf (Fig. 2). Therefore, equation (7) can be rewritten as follows using the general equation of Lorentz distribution.

■NJX-”K/ [1+ + (f+Δfsfn2y
rf.

1)/Δf1)] (8) where K is a proportionality constant. (The 5in2πflt component amplitude Vt of the 8-in formula is 5in2πft=-1 and 5in2πft=1 2 2 2-2fΔf) -
Δf - / (Δf + f1 ±Δf2 + 2fΔf) 2222 +Δf ) -4f Δ)
(9) The output of the synchronous detection circuit 17 is proportional to this vfl. For example, if the spectral line width Δf1 of the semiconductor lasers 1 and 10 is 30MH2, and the fi large frequency deviation Δf is 30MH2.
2, and f is -500 to 500MHz! The synchronous detection output vfIl when swept is as shown in FIG.

If the oscillation frequency of the semiconductor laser 10 is controlled to the 0 cross point a of the synchronous detection output in FIG. 3, then t=r1-f2-
f3=O・(10) That is, f1f2=f3...(11), so
The beat frequency matches the microwave frequency of the oscillator 15. That is, as shown in FIG. 4, the center frequency f1 of the oscillation of the semiconductor laser 1 is controlled to the absorption line frequency with high accuracy and in a highly stable frequency state, and the microwave frequency f3 is also stable, so the oscillation frequency of the semiconductor laser 10 is 2 also has high stability.

According to the frequency-stabilized laser light source having such a configuration, the oscillation frequency of the first semiconductor laser is controlled to the absorption line frequency by current modulation, and the oscillation frequency of the second semiconductor laser is controlled by offset using microwaves. By doing so, it is possible to obtain a non-modulated optical output with high frequency stability.

In addition, by changing the microwave frequency, the optical output frequency can be swept with high stability and accuracy.
A 1-2 GH2 sweep is possible for 0'rHz.

Since the absorption line is controlled by current modulation, no optical path changes occur in the acousto-optic modulator, resulting in excellent frequency stability.

In the above embodiment, the reference frequency for coherent detection is f
l was used to control the 0 cross point of the first-order differential signal from 1 to 1, but using 3f as the reference frequency, the 0-cross point of the third-order differential signal
It can also be controlled at cross points. Generally, it is possible to control the zero cross point of the odd-order differential signal by using a reference frequency that is an odd multiple of f.

Furthermore, the same control can be achieved even if the signal of fi f2+f3 of the V output is used. In this case f
The frequency of the output light is higher by 2f3 than when using f2 f3 signals.

Also, if mixer 5 is omitted and f3=0, f 2
= f 1 can be stabilized. However, in this case, f2 frequency sweep is not possible.

FIG. 5 is a block diagram showing a second embodiment of the frequency stabilized laser light source according to the present invention. The same parts as in FIG. 1 are given the same symbols and the explanation is omitted. 19 is a third oscillator for modulation, and 20 is a modulation circuit that frequency-modulates the microwave output signal of the oscillator 15 with the output of the oscillator 19.

The mixer 16 mixes the output of the modulation circuit 20 and the output of the photodetector 14, and the synchronous detection circuit 17 performs synchronous detection using the output of the oscillator 19 as a reference signal. The other configurations are the same as in the case of FIG.

Since the microwave output from the oscillator 15 is frequency-modulated by the modulation circuit 20 at a modulation frequency of 1□2, the output of the mixer 16 and the -1 signal are modulated at fra2.

If the output of the mixer 16 is synchronously detected by 'n2 in the synchronous detection circuit 17 and controlled so that the output becomes O, the beat frequency between the semiconductor lasers 1 and 10 becomes equal to the microwave center frequency.

According to the frequency stabilized laser light source having such a configuration, the frequency shift for controlling the oscillation frequency of the semiconductor laser 1 to the absorption line frequency by double modulation and the semiconductor laser 10e. 'f' Hirontai 1. -The 1 oscillation frequency 1
Since the 1J offset (and the 1z frequency deviation are independent), the optimum value can be selected for each (
Point 1 is less effective when the stability and S/N are significantly different from each other in terms of the spectral linewidth of the total radiation of the hJj absorption beam and the 4'# body L7~.

Also semiconductor 1. -9-ri'10! In a control system in which the oscillation frequency of the 1" conductor L-1 is controlled by adding an off-celler 1-1, the modulation is suppressed;
1' conductor laser]'s oscillation frequency to the absorption line frequency.
It is possible to improve the stability of the control system without changing the control system for a while.
To control the third-order differential, we use the reference frequency C1, 3f, 3f, , 2 to
It is also possible to control the O or D point of the 1' differential. Generally f
It is possible to control the odd-order differential signal to the zero point l= by using the odd-numbered reference frequency of n'lfl2.

Further, the modulation circuit 20 may have zero phase modulation.

2,i,,-,,,-,ni,:, iiKono @ JR sml t=, S ite,'-r'4 f) r= ly
- A part of the emitted light is taken as J-tengu light, and the part of the part is made into a thin beam from the opposite direction, and is absorbed by Nil-3. I5 to obtain saturated absorption (r'+ ``'j #l!l Sum absorption method〈Hori 5 Tsunoda 1 Kitano, Kazuzaki. Ogawa 9 Using saturated absorption spectroscopy ゛r
Conductor l/sasa frequency 7, electrification, telecommunications 1, k report OQ E
82-116), the stable frequency constant 1. - Realize the source: J can do it.

・Effects of S invention ・ According to the present invention, according to the present invention, , 1 also has (
Frequency is high stability, second control, high frequency stabilization.
It is possible to create a device that realizes a complex product of light afa.

4.) Brief explanation of FfJj Figure 1 shows the first embodiment of the frequency stabilized -17F light source according to the present invention. Fig. 1 shows the operation of the device; Fig. 55 shows a second embodiment of the frequency-stabilized light source according to the present invention; Fig. 1 shows the operation of the device; FIG. 6 is an elliptical diagram showing a conventional example of a light source with a circumferential power.

DESCRIPTION OF SYMBOLS 1... First semiconductor laser, 3... Absorption cell,... First photodetector, 5... First synchronous detection circuit, 6
...mi control circuit, 8...first oscillator, 1o.
... No. 1 \' conductor L - the, 12... Light 'Y: 1
Aperture, 1.1...Second photodetector, 15...Second excavator/device, 16...Mijinza, 17...Second synchronous detection circuit, 18...Second Control of 11:1 circuit, 10...Third oscillator, 20...1111-way fl...
The oscillation frequency of the first semiconductor laser, f2...the oscillation frequency of the second semiconductor laser.

Claims (2)

[Claims] (1) A first semiconductor laser, an absorption cell filled with a substance that absorbs the output light of the first semiconductor laser at a specific frequency, and a first semiconductor laser that detects the light transmitted through the absorption cell. a photodetector; a first oscillator that modulates the current of the first semiconductor laser;
a first synchronous detection circuit inputting the output of the photodetector, and a first synchronous detection circuit controlling the oscillation frequency of the first semiconductor laser to the absorption line of the absorption cell based on the output of the first synchronous detection circuit. a control circuit, a second semiconductor laser, and the first control circuit.
, an optical means for multiplexing the output light of the second semiconductor laser;
a second photodetector that detects the beat signal of the output light output from the optical means; a second oscillator; and a mixer that mixes the output of the second oscillator and the output of the second photodetector. a second synchronous detection circuit that synchronously detects the output of the mixer using the output of the first oscillator or a signal of an odd multiple frequency thereof as a reference signal; a second control circuit that controls the oscillation frequency of the second semiconductor laser such that the difference between the oscillation frequency of the first semiconductor laser and the oscillation frequency of the second semiconductor laser is equal to the oscillation frequency of the second oscillator; A frequency-stabilized laser light source characterized by being configured such that the output is unmodulated. (2) A third oscillator and the output of the second oscillator
A modulation circuit that performs frequency modulation or phase modulation with the output of the third oscillator, and a mixer mixes the output of the modulation circuit and the output of the second photodetector, and outputs the output of the third oscillator or a signal of an odd multiple thereof. 2. The frequency-stabilized laser light source according to claim 1, wherein the frequency-stabilized laser light source is used as a reference signal for the second synchronous detection circuit.