JP2578271Y2 - Frequency stabilized laser light source - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a frequency-stabilized laser light source used as an optical frequency reference of a coherent optical communication measuring instrument, and particularly to facilitate evaluation of stability by a beat method. The present invention relates to a frequency stabilized laser light source.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram showing a conventional example of a frequency-stabilized laser light source, in which a semiconductor laser is directly modulated and its oscillation frequency is controlled at the center of an atomic or molecular absorption line. Is shown. The output light of the semiconductor laser 1 is converted into parallel light by a lens 2 and then split into two directions by a beam splitter 3. One light is condensed on a fiber 4 by a lens 2 and becomes an output light. Is incident on the absorption cell 5 in which the standard substance is sealed. The light transmitted through the absorption cell 5 is detected by a photodetector 6, converted into an electric signal, and input to a synchronous detection circuit 7 including a lock-in amplifier and the like. The oscillation frequency of the semiconductor laser ₁ is current-modulated by the output of the oscillator 10 having the frequency f1, and the synchronous detection circuit 7 performs synchronous detection using the output of the oscillator 10 as a reference signal. The PI control circuit 8 controls the current of the semiconductor laser 1 so that the output of the synchronous detection circuit 7 becomes constant. The output of the PI control circuit 8 and the oscillation output of the oscillator 10 are added by the adder 9 and input to the semiconductor laser 1. As a result, the oscillation frequency of the semiconductor laser 1 is controlled to the center of the absorption line of the atoms or molecules of the standard substance of the absorption cell 5, and the laser output light has a high-precision absolute value determined by the atoms or molecules. Become.

[0003]

The evaluation of the stability of the light source as described above utilizes the beat signals of two light sources, and is performed by the configuration shown in FIG. The output light of the first light source 21 having the oscillation frequency f ₀₁ is synthesized with the output light of the second light source 22 having the oscillation frequency f ₀₂ shifted by the frequency f _s by the frequency shifter 23, and a photodiode (hereinafter referred to as “photo diode”). The beat signal detected at (PD) 25 is counted by the counter 26.

However, since the output light at the light source of FIG. 3 is modulated at the modulation frequency f _1, bi of the two light sources as shown in FIG. 4 - When evaluating the stability Use and signals, two bi a slight difference of the modulation frequency f ₁ of the - shift DOO frequency gradually, like a slow drift to the maximum frequency shift, there is a problem that the stability of the light source on the apparent deteriorates. The reason will be described below. The frequency f ₀ of the output light from the semiconductor laser 1 has a stable center frequency f _av as shown by the following equation, but instantaneously changes in a sinusoidal manner at the frequency f ₁ . f ₀ = f _av + f _div sin (2πf ₁ t) (1) Here, in the 1.5 μm band, f _av is about 200 THz,
f ₁ is usually 1~2kHz, the modulation amplitude f _div 2~500
The stability of MHz and f _av is about 1M when controlled by absorption line.
Hz. The output frequency f _beat of the PD 25 in FIG. 4 is represented by the following equation, where the center frequencies of the light sources 21 and 22 are f _av1 and f _av2 , and the modulation frequencies are f ₁₁ and f ₁₂ , respectively. _{f beat = f 01 -f 02 +} f s = {f av1 + f div sin (2πf 11 t)} - {f av2 + f div sin (2πf 12 t)} + f s = f av1 -f av2 + f s + 2f div cos { _{π (f 11 + f 12)} t} sin {π (f 11 -f 12) t} ... (2) where heterodyne frequency f _s is 80～140MHz. f _av1 −f _av2 is f when no modulation is applied.
_{The beat} is shown, but if f ₁₁ −f ₁₂ is very small, f _beat fluctuates with an amplitude of 2 f _div in that cycle, so that f
_It is measured that the stability of _{av1− fav2} is poor.
FIG. 5 is a diagram showing a state of frequency fluctuation when f ₁₁ and f ₁₂ are approximately the same, 2.083 kHz.

In order to avoid the above problem, it is conceivable to synchronize the modulation frequencies of the two light sources. However, since the phase delay of the semiconductor laser changes with time, perfect synchronization cannot be achieved. Can not. Therefore, it is difficult to evaluate the stability of the light source of FIG. 3 by the above method. The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to realize a frequency-stabilized laser light source whose output is frequency-modulated and whose stability can be easily evaluated by a beat method. I do.

[0006]

According to the present invention, there is provided a frequency-stabilized laser light source which directly modulates a semiconductor laser by an output of an oscillating means to control the oscillating frequency of the semiconductor laser at the center of an atomic or molecular absorption line. The oscillation means for switching the modulation frequency of the semiconductor laser by switching the frequency is provided.

[0007]

The oscillation means can switch a plurality of oscillation frequencies.
The stability can be accurately evaluated by shifting the modulation frequency of the two light sources.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of a frequency-stabilized laser light source according to the present invention. The same components as those in FIG.

In FIG. 1, reference numerals 10 and 11 denote _first and second oscillators having frequencies f ₁ and f ₂ , respectively.
A switch which selects one of the outputs 0 and 11 and applies it to the lock-in amplifier 7 as a reference frequency signal and adds it to the adder circuit 9 as a modulation signal. These switches constitute an oscillating means capable of switching between two oscillation frequencies. . Where f ₁ −f
_The modulation frequencies f ₁ and f ₂ are set so that ₂ is several tens Hz or more. Using the light source ₁ for which the modulation frequency f ₁ is selected by using the switch 12 and the light source ₂ for which the modulation frequency f ₂ is also selected, the stability of the beat frequency is evaluated in the configuration shown in FIG. _{Since 1−} f ₂ is several tens Hz or more,
As is clear from the equation (2), the frequency fluctuation is averaged by the counter 26. Therefore, accurate measurement can be performed without deteriorating the apparent stability. FIG. 2 shows that f ₁ = 2.00
This shows that the change in the frequency drift Δf when the modulation frequency is shifted to 0 kHz and f ₂ = 2.083 kHz is very small.

According to the frequency-stabilized laser light source having such a configuration, the stability can always be accurately evaluated by enabling the switching of the modulation frequency included in the output light. Instead of using the two oscillators 10 and 11 as the oscillating means, an oscillator that changes the oscillation frequency by switching the value of inductance or capacitance by the switch 12 or a VC that changes the oscillation frequency by switching the control voltage by the switch 12
O (voltage controlled oscillator) or the like may be used. Further, three or more kinds of modulation frequencies may be switched if necessary, for example, to obtain an appropriately shifted modulation frequency.

[0011]

As described above, according to the present invention, a plurality of modulation frequencies can be switched, so that the output is frequency-modulated, and the stability of the frequency can be easily evaluated by the beat method. -The light source can be realized with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a frequency-stabilized laser light source according to the present invention.

FIG. 2 is a characteristic curve diagram showing the stability of the apparatus of FIG.

FIG. 3 is a configuration block diagram showing an example of a conventional frequency stabilizing laser light source.

FIG. 4 is a configuration block diagram showing a method for evaluating the stability of a light source by a beat method.

FIG. 5 is a characteristic curve diagram showing the stability of a conventional frequency stabilizing laser light source.

[Explanation of symbols]

1 semiconductor laser - The 10 first oscillator 11 second oscillator 12 switches f _1, f ₂ the oscillation frequency

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tetsu Yoshitake 2-9-132 Nakamachi, Musashino City, Tokyo Yokogawa Electric Corporation (56) References JP-A-2-7587 (JP, A)

Claims (1)

(57) [Scope of request for utility model registration] 1. A frequency-stabilized laser light source that directly modulates a semiconductor laser by an output of an oscillating means and controls the oscillation frequency of the semiconductor laser at the center of an absorption line of an atom or a molecule. A frequency-stabilized laser light source comprising the oscillation means for switching a modulation frequency of a laser.