JPH10107377A - Method and device for generating wavelength-variable laser beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate wavelength-variable laser beams by setting the amount of translation movement of a movable diffraction grating corresponding to the length of an external resonator and the rotating angle of a movable diffraction grating, so that a specific equation can be met among the external resonator length, the amount of drive current change of a laser diode, a laser oscillation wavelength, and the groove gap of a movable diffraction grating. SOLUTION: The amount of translation movement corresponding to an external resonator length and the rotary angle of a movable diffraction grating are set, so that an equation can be met among the external resonator length, an amount of drive current change of a laser diode, a laser oscillation wavelength, and the groove gap of the movable diffraction grating, thus generating wavelength variable laser beams. In the equation. λ0 indicates the laser oscillation wavelength; L2 indicates the external resonator length; ΔL2 indicates the amount of translation move corresponding to the external resonator length L2 ; Δθ indicates the rotary angle of the movable diffraction grating, ΔI indicates the amount of drive current change for the laser diode; βindicates the proportionality constant that is specific to the laser diode element for determining the amount of change of wavelength with respect to the amount of drive current change of the laser diode; and d indicates the groove gap of the movable diffraction grating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable laser device suitable for use in isotope separation, atmospheric environment measurement, spectroscopic measurement, and the like.

[0002]

2. Description of the Related Art In general, semiconductor lasers are small in size, have extremely high oscillation efficiency, and are often used as light sources for optical communication and optical information processing.
aserDiode (LD).

[0003] This laser diode is capable of single mode oscillation and continuous wavelength sweeping by using only the diode alone. A precise tunable laser device using a laser diode having such characteristics is widely used in the field of optical communication. A laser diode used in the field of such optical communication oscillates laser light in a long wavelength range (1.3 μm to 1.5 μm). As such a laser diode, a distributed feedback (DFB) laser having a periodic structure in the resonator direction,
Distributed Bragg reflector type with similar periodic structure
ted Bragg Reflector (DBR) laser. The laser diode having the above-described structure can be used directly for wavelength control. Also, laser diodes are readily available.

[0004] By the way, in the laser light application technical fields such as isotope separation, atmospheric environment measurement, and spectroscopic measurement, a precise tunable laser device is required as described above. And
It is desirable to apply the laser diode to such a wavelength tunable laser device from the viewpoint of miniaturization and cost reduction.

However, laser diodes that can be used in the above-mentioned laser light application technical field have a short wavelength range (up to 0.6).
μm). At present, laser diodes capable of oscillating such laser light are limited to devices having a normal Fabry-Perot structure. The oscillation spectrum of a single laser diode having such a resonator structure generally has a large line width and multi-mode oscillation as compared with a DFB laser or a DBR laser having a periodic structure.

On the other hand, there has been provided a laser device in which an external resonator structure is added to a laser diode. In this laser device, narrowing of the spectrum and single mode oscillation are performed using an external resonator structure. In a laser device, narrowing the spectrum and performing single mode oscillation using an external resonator structure has been known as a most common wavelength control method for a laser diode. Among such wavelength control methods, a laser device that employs a wavelength control method using a diffracted light of a diffraction grating (hereinafter, referred to as “grating”) will be described with reference to FIGS.

FIG. 6 shows a conventional tunable laser device provided with a retro type external resonator. Tunable laser device 11
Reference numeral 0 denotes a laser diode 130 and a retro-type external resonator 150. Retro-type external resonator 15
Numeral 0 denotes a collimator lens 151 that converts the laser light emitted from the laser diode 130 into a parallel beam, an etalon 152 inserted into a path of the parallel beam from the collimator lens 151, and receives a parallel beam that has passed through the etalon 152. And a movable grating 153 that outputs an output laser beam. Here, grooves 154 are formed in the movable grating 153 at regular intervals. Further, in the conventional wavelength tunable laser device 110 using the retro type external resonator 150, the laser diode 13
For the purpose of sweeping a continuous wavelength, a non-reflection coating (anti-reflection coating: hereinafter referred to as an AR coating) 132 is applied to the laser beam emitting end face 131 of the laser diode 130 from a design concept of treating only a single element as a laser medium.

The operating characteristics of the wavelength tunable laser device 110 having such a retro-type external resonator 150 will be described with reference to FIG. In this figure, the horizontal axis represents frequency, and the vertical axis represents gain and the like. Laser diode 13
0 indicates a continuously wide gain curve 170 in a predetermined wavelength range (up to 20 nm). In the wavelength tunable laser device 110, the longitudinal mode 171 of the retro-type external resonator 150 is generated at predetermined frequency intervals. Also,
Reference numeral 172 indicates the resolution of the movable grating 153, which includes seven longitudinal modes of the retro-type external resonator 150. Therefore, in order to obtain single mode oscillation, an etalon 152 is inserted as a wavelength selection element. The resolution of the etalon 152 includes only one external resonator longitudinal mode, and the mode becomes a single mode oscillation.

As described above, the retro-type external resonator 150 is used.
According to the conventional wavelength tunable laser device 110 provided with the above, by inserting the movable grating 153 and the etalon 152, the spectrum narrowing of the laser light and the single mode oscillation are performed.

On the other hand, in the wavelength tunable laser device basically provided with the above-mentioned retro type external resonator, the laser light emitting end face of the laser diode 130 is not coated with an AR coating.
A tunable laser device that does not require the insertion of the etalon 152 has also been proposed. This laser device has been proposed by solving the following disadvantages as follows.

That is, in general, in a laser device having this type of retro-type external resonator 150, the length of the external resonator 150 and the movable grating
When the angle 3 is changed, there is usually a disadvantage that the laser light output becomes unstable and mode hop occurs. This laser device is configured to monitor the laser beam output and control the drive current of the laser diode so that the laser beam output value is maximized, so that continuous wavelength sweeping can be performed. Continuous wavelength sweep is performed by controlling the drive current of the laser diode by using a complicated device such as a sensor for detecting the laser light output, a local oscillator, a lock-in amplifier, and an analog control circuit.

FIG. 8 shows a conventional wavelength tunable laser device having a Littman type external resonator. The wavelength tunable laser device 210 includes a laser diode 230 and a Littman type external resonator 250. The Littman type external resonator 250 is a collimating lens 2 that converts the laser light emitted from the laser diode 230 into a parallel beam.
51, a fixed grating 253 that receives the parallel beam from the collimating lens 251 to reflect and output laser light, and a fixed grating 2 that reflects the laser light reflected from the fixed grating 253 and reflects the laser light again.
And a movable mirror 252 to be provided to the light source 53. Here, the fixed grating 253 has grooves 2 at regular intervals.
54 are formed. Further, in the conventional wavelength tunable laser device 210 using the Littman type external resonator 250,
In order to continuously sweep the wavelength from the design concept of treating a single laser diode 230 only as a laser medium, the laser light emitting end face 231 of the laser diode 230 has an AR coating 2
32 is given.

According to the wavelength tunable laser device 210,
The curve 173 in FIG.
Thus, the spectrum narrowing of the laser light and the single mode oscillation are performed.

[0014]

[Problems to be solved by the invention]

(1) However, these external resonators 150, 250
In the conventional tunable laser devices 110 and 210 using the laser diode 13 for the above-described reason,
AR coat 132 on the laser light emitting end face
232 needs to be applied. (1-a) Therefore, there is a disadvantage that special equipment is required to apply the AR coating to the laser light emitting end faces of the laser diodes 130 and 230. (1-b) In addition to the need for the material of the AR coat,
Since a step of applying an AR coating is also required, there is a disadvantage that the cost of the laser diode increases. (1-c) Further, in the step of applying the AR coating, the laser diode is exposed to the atmosphere,
There is also the disadvantage that there is a risk of optical damage.

(2) In the case of the retro-type external resonator 150 shown in FIG. 6, the laser diode 130 has a continuously wide gain curve 170 in a predetermined wavelength range (up to 20 nm) as shown in FIG. Etalon 15 as a wavelength selection element having a high resolution 173
By inserting 2, the single mode oscillation was obtained. Therefore, according to the conventional wavelength tunable laser device 110 including the retro-type external resonator 150, the number of components of the optical system increases, and there is a disadvantage that adjustment is complicated.

(3) Further, in controlling the wavelength, the length of the external resonator and the diffraction angle of the movable grating 153 must be appropriately linked so far.
A special mechanism must be devised in which the rotation angle and the translation amount of the 3 (or the movable mirror 252 in the Littman type external resonator 250) are mechanically linked, and a mechanism for performing precise optical axis adjustment is also required. I had to take care.

(4) On the other hand, in the case of a wavelength tunable laser device in which the laser light emitting end face of the laser diode 130 is not provided with an AR coating and does not require the insertion of the etalon 152, it is necessary to control the laser diode driving current. ,
Local oscillator, lock-in amplifier, analog control circuit, etc.
Complicated equipment is required, and it is necessary to achieve lower cost for practical use.

An object of the present invention is to provide a small-sized and low-cost tunable laser light generating method and apparatus for isotope separation, atmospheric environment measurement, spectroscopic measurement, and the like.

[0019]

In order to achieve the above object, according to the first aspect of the present invention, there is provided a wavelength tunable laser light generating method, wherein a laser diode for oscillating laser light and an external resonator including a movable diffraction grating are provided. Wherein the translation amount of the movable diffraction grating and the rotation angle of the movable diffraction grating with respect to the length of the external resonator are controlled by controlling the length of the external resonator and the driving of the laser diode. By setting the amount of change in the current, the laser oscillation wavelength, and the interval between the grooves of the movable diffraction grating so as to satisfy Expression 3, it is possible to generate a wavelength-variable laser beam.

[0020]

ΔL ₂ = L ₂ · β · ΔI / λ ₀ Δθ = sin ⁻¹ [(λ ₀ + β · ΔI) / 2d] −sin ⁻¹ (λ
₀ / 2d) where λ ₀ is the laser oscillation wavelength, L ₂ is the external resonator length,
ΔL ₂ is the translational movement amount with respect to the external resonator length L ₂ , Δθ is the rotation angle of the diffraction grating, ΔI is the change amount of the drive current of the laser diode, and β is the laser diode element that determines the change amount of the wavelength with respect to the drive current of the laser diode. The inherent proportionality constant, d, is the groove spacing of the diffraction grating.

Therefore, according to the first aspect of the present invention, the structure of the retro-type external resonator in which the AR coating is not applied to the emission end face of the laser diode does not require the insertion of an etalon, and uses only a collimating lens and a movable grating. Since the system is configured, the simplest, compact, and low-cost tunable laser light can be obtained.

According to the wavelength tunable laser device according to the second aspect of the present invention, in a laser device including a laser diode that oscillates a laser beam and an external resonator including a movable diffraction grating, the rotation of the diffraction grating is controlled. A first actuator that can set an angle, a second actuator that can set a translation amount of a diffraction grating, a control power supply that can set and control a drive current value of a laser diode, a first actuator, a second actuator, and control And control means for controlling the driving of each power supply.

Therefore, according to the second aspect of the present invention, A
The structure of a retro-type external resonator that does not apply the R coat to the emission end face of the laser diode does not require the insertion of an etalon,
Since the optical system is composed of only the collimator lens and the movable grating, the simplest, compact, and low-cost tunable laser device can be obtained.

According to the third aspect of the present invention, the external resonator includes only a collimating lens that emits a parallel beam from the laser diode and a movable diffraction grating that receives the parallel beam from the collimating lens and outputs the light as output light. It consists of.

Further, in the invention according to claim 4, the control means comprises: a laser oscillation wavelength of λ ₀ , a translation amount with respect to the external resonator length L ₂ of ΔL ₂ , a rotation angle of the diffraction grating of Δθ, a driving of the laser diode. Assuming that the amount of change in current is ΔI, the proportional constant inherent to the laser diode element that determines the amount of change in wavelength with respect to the drive current of the laser diode is β, and the groove interval of the diffraction grating is d, the relationship of Equation 4 is satisfied. Processing means for controlling the driving of each of the first actuator, the second actuator, and the control power supply is provided.

[0026]

ΔL ₂ = L ₂ · β · ΔI / λ ₀ Δθ = sin ⁻¹ [(λ ₀ + β · ΔI) / 2d] −sin ⁻¹ (λ
₀ / 2d) Further, in the invention according to claim 5, a temperature sensor is provided in the laser diode, and a detection signal from the temperature sensor is fed back to a temperature controller of the laser diode, so that the first actuator, the second actuator, The control means of the control power supply controls the temperature of the laser diode.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the wavelength tunable laser device according to the present invention will be described below.

[First Embodiment] FIG. 1 shows a first embodiment of a tunable laser device according to the present invention. In this figure, a tunable laser device 1 controls a laser diode 3 that oscillates a laser beam, an external resonator 5 added to the laser diode 3, a driving current of the laser diode 3, and a resonance state of the external resonator 5. Controlled means 7
And a control means 9 for controlling the driving of the controlled means 7. The laser diode 3 is maintained at a constant temperature by the temperature controller 2 which drives the Peltier element 21 by the output from the temperature sensor 95. Reference numeral 22 denotes a heat sink.

Here, the external resonator 5 includes a collimator lens 51 that converts the laser light emitted from the laser diode 3 into a parallel beam, and a movable diffraction grating (which receives the parallel beam from the collimator lens 51 and outputs the light). (Movable grating) 53. Further, grooves 54 are provided in the movable grating 53 at a constant interval d.

The controlled means 7 includes a first actuator 7
1, a second actuator 72, and a control power supply 73.

The first actuator 71 can set the rotation angle Δθ of the movable grating 53. Although not shown, the first actuator 71 is composed of, for example, a piezoelectric element and a mechanical system, and the piezoelectric element is accurately displaced and stopped at a position determined by the applied voltage, so that the action of the mechanical system is achieved. Movable grating 53
Can be set to an accurate rotation angle Δθ.

The second actuator 72 can set the translation amount ΔL ₂ of the movable grating 53.
Although not shown, the second actuator 72 includes, for example, a piezoelectric element and a mechanical system. A first actuator 71 is fixed on the mechanical system. Therefore, when the piezoelectric element of the second actuator 72 is accurately displaced and stopped by the position determined by the applied voltage, the movable grating 53 including the first actuator 71 can be accurately translated and moved in conjunction with the action of the mechanical system. ΔL ₂ can be set.

The laser diode 3 is connected to a control power source 7
3 is connected. The control power supply 73 can be set the drive current I _N flowing to the laser diode 3 in accordance with a control signal. The control means 9 includes a first actuator 71,
It is electrically connected to the second actuator 72 and the control power supply 73, and can output each control signal to the first actuator 71, the second actuator 72, and the control power supply 73.

The control means 9 includes a processing device main body 91,
A keyboard 92 is provided as input means for designating constants, processing methods, and the like, and a display 93 for displaying processing results. Although not shown, the processing device main body 91 includes, for example, an arithmetic processing unit that executes various types of arithmetic processing, a read-only memory that stores a series of processing programs to be processed by this device, and various data and the like in performing the processing. And an interface for outputting a control signal or the like to the outside or taking in data from the outside. Further, the read-only memory stores the relationship between the control signal output to the first actuator 71 and the second actuator 72 of the processing device main body 91 and the actual rotation angle and moving distance of the movable grating 53. Similarly, the read-only memory stores the relationship between the control signal output from the processing apparatus main body 91 to the control power supply 73 and the drive current supplied to the laser diode 3 by the control power supply 73. The processing device main body 91 can read the data of the actual rotation angle, movement amount, or drive current of the grating from the read-only memory based on the output control signal, and use it for control.

FIG. 2 shows a wavelength tunable laser device 1 having the above configuration.
FIG. In this figure, reference numerals 31, 32
Is an end face of the laser diode 3. Reference numeral L ₁ is a length between the end faces 31, 32 of the laser diode 3, a resonator length comprised between the end faces 31 and 32. Note that L
Reference numeral ₁ denotes an optical length in consideration of a refractive index of a medium, not a mechanical dimension between end faces of the laser diode 3.
The number of longitudinal modes existing between the end faces 31 and 32 is N
_Set to ₁ . Next, reference numeral L ₂ is a distance from the emission end face 31 of the laser diode 3 to the movable grating 53, a resonator length comprised between the exit end face 31 and the movable grating 53 of the laser diode 3. The number of longitudinal modes between the emission end face 31 and the movable grating 53 is N ₂ , and the laser oscillation wavelength of the laser diode 3 is λ ₀ . The symbol d represents the movable grating 5
3 is the interval between the grooves 54, and the symbol θ is the incident angle of the laser beam on the movable grating 53.

Here, in order to shift the laser oscillation wavelength λ ₀ by Δλ without causing a mode hop, while the wavelength shifts, the number of longitudinal modes N ₁ and N _{2 of the two} resonators are changed.
Should always be constant. Therefore, the following formulas will be summarized, including the conditions under which the primary diffraction light of the movable grating 53 returns to the laser diode 3.

(I) At λ ₀ Between the end faces 31 and 32 of the laser diode 3,
Holds.

[0038]

Equation 5] Also _{L 1 = N 1 · λ 0} /2, in between the emission end face 31 and the movable grating 53 of the laser diode 3, Equation 6 is satisfied.

[0039]

[6] _{L 2 = N 2 · λ 0} /2 Further, the incident angle of the laser beam θ of the movable grating 53, the groove spacing d of the movable grating 53, between the laser oscillation wavelength lambda _0, Equation 7 Holds.

[0040]

## EQU7 ## 2 d sin θ = λ₀ (ii) and λ₀+ Δλ Between the end faces 31 and 32 of the laser diode 3, a resonator
Length L₁ΔL ₁Then, Equation 8 is established.

[0041]

L ₁ + ΔL ₁ = N ₁ · (λ ₀ + Δλ) / 2 Further, between the emission end face 31 of the laser diode 3 and the movable grating 53, the change in the resonator length L ₂ is represented by ΔL
Assuming ₂ , Equation 9 holds.

[0042]

L ₂ + ΔL ₂ = N ₂ · (λ ₀ + Δλ) / 2 Further, the angle (θ + Δθ) of the movable grating 53
, The groove interval d of the movable grating 53, and the wavelength (λ ₀
+ Δλ), Equation 10 holds.

[0043]

From the above, 2dsin (θ + Δθ) = (λ ₀ + Δλ) From the above, the change Δ of the _two resonator lengths L ₁ and L ₂
When the relationship between L ₁ and ΔL ₂ is obtained, Equation 11 is obtained.

[0044]

ΔL ₂ = ΔL ₁ × L ₂ / L ₁ On the other hand, the relationship between the change Δλ in the laser oscillation wavelength and the change ΔI in the drive current of the laser diode 3 is as shown in Expression 12.

[0045]

Δλ = β · ΔI The sign β is a change in wavelength with respect to the laser diode driving current.
It is an inherent proportional constant of the laser diode 3 that determines
You. The change ΔI in the drive current of the laser diode 3 is
ΔL through Equations 12 and 8₁Works with
ΔL through 11 _TwoIt is linked with. ΔI is a number
Also linked with Δλ and Δθ via Equations 12 and 10.
You.

Therefore, the rotation angle Δ of the movable grating 53 necessary to shift the laser oscillation wavelength λ ₀ by Δλ
θ and the translation amount ΔL ₂ of the movable grating 53 by Δ
When expressed using I, Expression 13 and Expression 14 are obtained.

[0047]

ΔL ₂ = L ₂ · β · ΔI / λ ₀

[0048]

Δθ = sin ⁻¹ {(λ ₀ + β · ΔI) / 2d}
−sin ⁻¹ (λ ₀ / 2d) Therefore, the constant β, the groove interval d of the movable grating 53, the original oscillation wavelength λ ₀ of the laser diode 3, and the external resonator length L ₂ are determined from the keyboard 92. The first actuator 71, the second actuator 72, and the control power supply 73 are controlled so as to satisfy Expressions 13 and 14 in the processing device main body 91.

Since the wavelength tunable laser device 1 is controlled electrically, it is possible to design and manufacture the laser device free from the structural restriction of mechanical interlocking unlike the conventional laser device.

[Second Embodiment] FIG. 3 shows a second embodiment of the wavelength tunable laser device. In this figure, in the second embodiment, a temperature sensor 95 for detecting the temperature of the laser diode 3 in the wavelength tunable laser device 1 'and converting it into an electric signal is provided, and the output from the processing device main body 91 is converted to a temperature. Input to the controller 2 and the laser diode 3
The first actuator 71, the second actuator 72, and the control power supply 73 are controlled so as to satisfy Expressions 13 and 14 in consideration of the operating temperature of the above. Therefore, the other configuration is exactly the same as that of the wavelength tunable laser device 1 of FIG. 1, and the same reference numerals are given and the description of the configuration and operation is omitted.

According to the second embodiment, since the operating temperature of the laser diode 3 is taken into control parameters and controlled, a wider wavelength range can be covered.

[Third Embodiment] FIG. 4 shows a third embodiment of the wavelength tunable laser device. The control system of the wavelength tunable laser device 1 ″ in this figure employs a self-tuning control method, and has a configuration different from that of the open loop control method such as the wavelength tunable laser devices of FIGS. 1 and 3. That is, in the wavelength tunable laser device 1 ″ according to the third embodiment, the current detection sensor 81 that detects a current supplied from the control power supply 73 to the laser diode 3 and converts the current into an electric signal is connected to the control power supply 73 and the laser diode. 3, a point at which a rotation angle detection sensor 82 that detects the rotation angle of the movable grating 53 and converts it into an electric signal is provided on the rotation axis of the movable grating 53, and a translation amount of the movable grating 53. The translational movement detecting sensor 83 for detecting the movable grating 5
First, there is a feature in the point provided in 3. These sensors 81, 8
The detection signals from the control unit 9 and the processing unit 9
1 is input. Further, the processing apparatus main body 9
In FIG. 1, the first actuator 71, the first actuator 71,
The second actuator 72 and the control power supply 73 are controlled. This also allows generation of a tunable laser beam.

According to the third embodiment, the displacement characteristics of the piezoelectric element and the proportional constant β of the laser diode, which are internal variables, can be estimated from the relationship between the control signal, the rotation angle, the amount of movement, and the drive current. Then, these internal variables can be adjusted using a sequential equation based on the least squares method so as to minimize the estimation error, and it is possible to absorb minute individual differences of hardware.

[Fourth Embodiment] Next, a fourth embodiment will be examined as a simple example. In the fourth embodiment, when Δθ〜0, Expression 15 is satisfied.

[0055]

ΔL ₂ / Δθ = L ₂ / tan θ = constant Therefore, by determining an appropriate fulcrum, the rotation angle and translation amount of the movable grating can be controlled by one actuator. △ I at this time
From Expressions 13 and 15, △ I = (λ ₀ / βtan θ)
Can be written as Δθ.

As a result of conducting an experiment using the equation (15),
20 [GH] centering on the oscillation wavelength λ ₀ = 628 [nm]
z] The continuous wavelength sweep as described above was confirmed. At this time, the set parameters are: L ₂ = 26 [mm], β = 5 × 10
^-3 [nm / mA], ΔI = 7.5 [mA], d = 1/1
800 [mm]. The operating temperature of the laser device is 25.00 [° C.]. In the experiment, FSR
= 2 [GHz] and an optical spectrum analyzer with a finesse of 300 were used as measuring instruments.

In the fourth embodiment, the expression 15
Is used, there is an upper limit to the continuous sweep width because of structural restrictions as shown below. This will be described with reference to FIG.

As an initial state, the laser diode has a wavelength λ.
Oscillation in single mode at ₀ , L ₂ / tan θ = r
Suppose (constant) holds. That is, since L ₂ is the line segment AC, θ is ∠ABC, and r is the line segment BC, this represents a geometric structure formed by the triangle ABC.

Next, θ → θ + Δθ, L ₂ → L ₂ + Δ
Consider the case where L ₂ , λ ₀ → λ ₀ + Δλ.

At this time, ΔL ₂ / Δθ = r (constant), ΔL ₂ is a line segment ON, Δθ is ∠OA'N, and r is a line segment A'O, so that the movable grating Can be considered to have moved to the point A ′.

On the other hand, (L ₂ + ΔL ₂ ′) / tan (θ + Δ
θ) = r (constant) holds, and ΔL ₂ ′ / Δθ =
r / cos ² θ holds. Here, (θ + Δθ) is ∠
Since A is "B'C" and r is equivalent to the line segment B'C ", it is necessary to consider that point A has moved to point A" in order to establish the formula.

As described above, in the approximate expression for Δθ to 0, the interpretation of the moving point of point A is expressed by δ (Δ
L ₂ ) will cause ambiguity.

[0063]

Δ (ΔL ₂ ) = ΔL ₂ ′ −ΔL ₂ = r · Δθ /
cos ² θ−r · Δθ δ (ΔL ₂ ) can be represented by a line segment NM.

However, in practice, considering the discrete structure of the movable grating, the ambiguity expressed by Expression 17 has no meaning when the groove interval is equal to or less than the groove interval of the movable grating. That is, if the movable grating has a periodic structure with the groove interval d, the condition of the continuous wavelength sweep width is as follows:

[0065]

| Δ (ΔL ₂ ) | <d · sin θ is a condition. Assuming that the upper limit value of the continuous wavelength sweep width is Δν _MAX and the amount of change in the movable grating angle at that time is Δθ _MAX , the following equation 18 can be used.

[0066]

Δν _MAX = (c · r / λ ₀ · L ₂ ) · Δθ _MAX = (c · r / λ ₀ · L ₂ ) (2d · cos ² θ / r · sin θ) = c / (L ^₂ · tan _{2 θ)} ≒ 24 [GHz] this value and the experimental result well be explained, in the simple fourth embodiment using a single actuator, it can be seen that there is a limit to the wavelength control. However, since only one actuator is required, it is desirable to use the tunable laser device for a specific application.

The present invention is not limited to the above embodiments, and various modifications and changes can be made without departing from the spirit of the invention described in the appended claims.

[0068]

As described above, according to the wavelength tunable laser beam generating method according to the first aspect of the present invention, the translation amount of the movable diffraction grating and the rotation angle of the movable diffraction grating with respect to the length of the external resonator are determined. The length of the external resonator, the amount of change in the drive current for driving the laser diode, the laser oscillation wavelength, and the gap between the grooves of the movable diffraction grating can be set so as to satisfy Equation 19, so that an AR coat is formed on the emission end face of the laser diode. It is possible to use a retro-type external resonator structure without applying it, and it is not necessary to insert an etalon, and the optical system is composed only of a collimating lens and a movable grating. A tunable laser beam can be obtained.

[0069]

ΔL ₂ = L ₂ · β · ΔI / λ ₀ Δθ = sin ⁻¹ [(λ ₀ + β · ΔI) / 2d] −sin ⁻¹ (λ
₀ / 2d) where λ ₀ is the laser oscillation wavelength, L ₂ is the external resonator length,
ΔL ₂ is the translational movement amount with respect to the external resonator length L ₂ , Δθ is the rotation angle of the diffraction grating, ΔI is the change amount of the drive current of the laser diode, and β is the laser diode element that determines the change amount of the wavelength with respect to the drive current of the laser diode. The inherent proportionality constant, d, is the groove spacing of the diffraction grating.

Further, according to the wavelength tunable laser device according to the second aspect of the present invention, the first actuator capable of setting the rotation angle of the diffraction grating, the second actuator capable of setting the translation amount of the diffraction grating, and Since a control power supply that can set and control the drive current value of the laser diode and control means that can drive and control each of the first actuator, the second actuator, and the control power supply are provided, an AR coat is not applied to the emission end face of the laser diode. The simplest, compact, and low-cost wavelength can be configured as a retro-type external resonator, and the optical system can be configured with only a collimating lens and a movable grating without the need to insert an etalon. A tunable laser device can be obtained.

According to the third aspect of the present invention, since the external resonator is composed of only the collimating lens and the movable diffraction grating, the structure of the external resonator is simplified and the cost can be reduced.

According to the fourth aspect of the present invention, the control means controls the laser oscillation wavelength to λ ₀ , the translation amount relative to the external resonator length L ₂ to ΔL ₂ , the rotation angle of the diffraction grating to Δθ, and the drive current of the laser diode. When the amount of change is ΔI, the proportional constant inherent to the laser diode element that determines the amount of change in wavelength with respect to the drive current of the laser diode is β, and the groove spacing of the diffraction grating is d, the first equation is established so that the relationship of Expression 20 is established. The actuator, the second actuator, and a processing unit for controlling the driving of the control power supply.
Even for more practical designs such as automation of wavelength control, there is room for improvement items to be absorbed by software.
It can flexibly respond to application aspects.

[0073]

ΔL ₂ = L ₂ · β · ΔI / λ ₀ Δθ = sin ⁻¹ [(λ ₀ + β · ΔI) / 2d] −sin ⁻¹ (λ
₀ / 2d) In the invention according to the fifth aspect, a temperature sensor is provided in the laser diode, and a detection signal from the temperature sensor is fed back to a temperature controller of the laser diode, so that the first actuator, the second actuator, and the control power supply are provided. Since the temperature control of the laser diode is performed by the control means (1), more precise control becomes possible in controlling the wavelength.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a wavelength tunable laser device according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining an operation principle of the first embodiment.

FIG. 3 is a block diagram showing the second embodiment.

FIG. 4 is a block diagram showing the third embodiment.

FIG. 5 is a diagram for explaining the fourth embodiment.

FIG. 6 is a block diagram showing a conventional laser device.

FIG. 7 is a diagram for explaining the operation of the laser device.

FIG. 8 is a block diagram showing another conventional laser device.

[Explanation of symbols]

 1, 1 ', 1 "wavelength variable laser device 2 temperature controller 3 laser diode 31 emission end face 32 end face 5 external resonator 51 collimating lens 53 movable grating 54 groove 7 controlled means 71 first actuator 72 second actuator 73 Control power supply 81 Current detection sensor 82 Rotation angle detection sensor 83 Translation movement amount detection sensor 9 Control means 91 Processing unit main body 92 Keyboard 93 Display 95 Temperature sensor

Claims (5)

[Claims] 1. A method for generating tunable laser light by a laser device having a laser diode that oscillates laser light and an external resonator including a movable diffraction grating,
The translational movement amount of the movable diffraction grating and the rotation angle of the movable diffraction grating with respect to the external cavity length, the external cavity length, the drive current change amount for driving the laser diode,
A wavelength tunable laser light generating method characterized in that a wavelength tunable laser light can be generated by setting a value between a laser oscillation wavelength and a groove interval of the movable diffraction grating so as to satisfy Expression 1. ΔL ₂ = L ₂ · β · ΔI / λ ₀ Δθ = sin ⁻¹ [(λ ₀ + β · ΔI) / 2d] −sin ⁻¹ (λ
₀ / 2d) where λ ₀ is the laser oscillation wavelength, L ₂ is the external resonator length,
ΔL ₂ is the translational movement amount with respect to the external resonator length L ₂ , Δθ is the rotation angle of the diffraction grating, ΔI is the change amount of the drive current of the laser diode, and β is the laser diode element that determines the change amount of the wavelength with respect to the drive current of the laser diode. The inherent proportionality constant, d, is the groove spacing of the diffraction grating. 2. A laser device comprising: a laser diode that oscillates laser light; and an external resonator including a movable diffraction grating, a first actuator that can set a rotation angle of the diffraction grating, and a translation of the diffraction grating. A second actuator capable of setting a movement amount, a control power supply capable of setting and controlling a drive current value of the laser diode, and control means capable of controlling the drive of the first actuator, the second actuator, and the control power supply, respectively. A wavelength tunable laser device, comprising: 3. The collimator lens, wherein the external resonator emits light of the laser diode as a parallel beam,
3. The tunable laser device according to claim 2, further comprising a movable diffraction grating that receives a parallel beam from the collimator lens and outputs the received light. 4. The control means sets a laser oscillation wavelength to λ.
₀, External resonator length L _TwoΔL_Two, Times
The rotation angle of the folded grating is Δθ and the drive current of the laser diode is
The amount of change is ΔI, and the waveform is
Proportional constant inherent to laser diode element that determines length variation
Where β is the number and d is the groove interval of the diffraction grating,
The first actuator and the second actuator so that
Drive and control of the actuator and control power supply
3. The processing device according to claim 2, further comprising:
Tunable laser device. (Equation 2) ΔL_Two= L_Two・ Β ・ ΔI / λ₀ Δθ = sin^-1[(Λ₀+ Β · ΔI) / 2d] -sin^-1(Λ
₀/ 2d) 5. A laser diode is provided with a temperature sensor, and a detection signal from the temperature sensor is fed back to a temperature controller of the laser diode, and is controlled by the first actuator, the second actuator, and a control unit of a control power supply. 3. The wavelength tunable laser device according to claim 2, wherein the temperature of said laser diode is controlled.