JP2001144360A - Method and device for measuring threshold for oscillation of semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure accuracy of measurement of threshold for oscillation of a laser beam, even when the intensity of the laser beam is small and to eliminate the inability to recognize laser oscillation, even in the worst case and to determine the threshold for oscillation, even when the laser beam is not emitted to the outside. SOLUTION: A threshold for oscillation of semiconductor laser is measured, based on the change in a curvature of current-voltage characteristics of the semiconductor laser. It is measured, also based on the change in the micro coefficient of the current-voltage characteristics of the semiconductor laser.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring an oscillation threshold value of a semiconductor laser, and more particularly, to an element having a total reflection surface on an element side face or an end face and emitting no laser light outside the semiconductor laser. The present invention relates to a method and an apparatus for measuring a threshold value of laser oscillation.

[0002]

2. Description of the Related Art It is known that a semiconductor laser oscillates by light excitation or current injection excitation. Normal semiconductor lasers emit laser light to the outside,
The intensity of the laser light emitted outside is measured, and the point at which the slope efficiency or the external differential quantum efficiency sharply changes is defined as the oscillation threshold. Above this oscillation threshold, the spectral line width of the emitted light decreases, the emission angle decreases,
An improvement in the degree of polarization is observed.

On the other hand, it is difficult for a ring resonator type semiconductor laser or a disk laser having a total reflection surface on the element side surface to extract laser light to the outside. In an element in which laser light is positively confined inside the element and the interaction between the laser light and the medium inside the element is used, the efficiency is higher if the laser light is not emitted to the outside. However, the oscillation threshold of the semiconductor laser has been determined from the change in the light intensity of the laser light emitted to the outside.

[0004] The method of determining the oscillation threshold value from the change in the light intensity of the laser beam is already well known. For example, as a document, “Optical communication device engineering” (engineering book) by Hiroo Yonezu is known. is there.

[0005]

However, in the conventional method of measuring the oscillation threshold, the intensity of the laser light emitted to the outside is small, such as a ring resonator type semiconductor laser or a disk laser, or the laser When no light is emitted, there are the following disadvantages. That is, when the intensity of the laser light emitted to the outside is low, the S / N of the signal obtained from the photodetector is deteriorated, so that the measurement accuracy of the oscillation threshold value is low. I couldn't do that. When the laser light was not emitted to the outside, the oscillation threshold could not be determined.

In view of the above-mentioned conventional problems, the present invention makes it possible to accurately measure the oscillation threshold value even when the intensity of laser light emitted to the outside is low or no laser light is emitted to the outside. It is an object of the present invention to provide a method and an apparatus for measuring an oscillation threshold value of a semiconductor laser.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to measure an oscillation threshold of a semiconductor laser based on a change in curvature of a current-voltage characteristic of the semiconductor laser. Achieved by measuring the value.

An object of the present invention is to provide a semiconductor laser having a current
This is achieved by a method for measuring the oscillation threshold of a semiconductor laser, which comprises measuring the oscillation threshold of the semiconductor laser based on a change in the derivative of the voltage characteristic.

It is an object of the present invention to provide a semiconductor laser having a current
This is achieved by an apparatus for measuring the oscillation threshold of a semiconductor laser, comprising a circuit for calculating the oscillation threshold of the semiconductor laser based on a change in the curvature of the voltage characteristic.

An object of the present invention is to provide a semiconductor laser having a current
This is achieved by an apparatus for measuring the oscillation threshold of a semiconductor laser, which includes a circuit for calculating the oscillation threshold of the semiconductor laser based on the change in the derivative of the voltage characteristic.

An object of the present invention is to measure a laser oscillation threshold value in a ring laser comprising a semiconductor optical amplifying element and an optical fiber based on a change in a curvature or a derivative of a current-voltage characteristic of the semiconductor optical amplifying element. This is achieved by a method of measuring an oscillation threshold value of a semiconductor laser.

An object of the present invention is to calculate a laser oscillation threshold value in a ring laser comprising a semiconductor optical amplifying element and an optical fiber based on a change in a curvature or a derivative of a current-voltage characteristic of the semiconductor optical amplifying element. This is achieved by an apparatus for measuring the oscillation threshold of a semiconductor laser, characterized by including a circuit for performing the above.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings. First, the principle of the present invention that the oscillation threshold can be measured even when the intensity of the laser light emitted to the outside is low or the laser light is not emitted to the outside will be described using a rate equation.

Assuming that the photon number density inside the semiconductor laser is S and the carrier concentration is n, the rate equation is given by the following equation.

[0015]

(Equation 1)

[0016]

(Equation 2) Here, G (n) is a gain coefficient, β _S is a spontaneous emission light coupling coefficient, τ _r is a luminescence recombination lifetime, τ _ph is a photon lifetime, J is a current density, q is an elementary charge, and d is an active layer. The thickness, τ _n, is the carrier lifetime. Then, the gain coefficient G (n) is obtained by setting the carrier concentration at which the active layer becomes transparent to n ₀ .

[0017]

(Equation 3) It is expressed as

From equations (1) and (2), in the steady state,

[0019]

(Equation 4)

[0020]

(Equation 5) Becomes Diffusion current, drift current, and recombination current can be considered as the current flowing in the semiconductor laser. Here, only the dominant recombination current is considered, and other contributions are ignored.

The carrier concentration n is given by the following equation using the intrinsic carrier concentration n _i and the voltage V applied to the pn junction: n = n _i exp [qV / (2 k _B T)] (6) Where the intrinsic carrier concentration _ni is

[0022]

(Equation 6) It is. k _B is Boltzmann's constant, T is absolute temperature, and h is Planck's constant. Further, m _e, m _h respectively electrons,
The effective mass of holes, E _g, is the band gap energy of the active layer.

From the equations (4) to (6), the relational expression between the injection current I to the semiconductor laser and the applied voltage V is obtained as follows.

[0024]

(Equation 7) Here, _VA is the volume of the active layer, and the area of the active layer is S
_Assuming that _A , _VA = d × S _A (9)

The laser oscillation begins, the photon number density S of the semiconductor laser, abruptly increases as compared with the following values oscillation threshold I _th, current in accordance with Equation (8) - voltage characteristic changes. Therefore, by detecting a change in the curvature of the current-voltage characteristic, the oscillation threshold can be measured. Further, by finding the derivative of the current-voltage characteristic such as the differential resistance, it becomes even easier to detect a change in the curvature of the current-voltage characteristic.

FIG. 6A shows a current-voltage (IV) characteristic of a semiconductor laser, and FIG. 6B shows a current-differential resistance (Id).
V / dI) characteristic, FIG. 6C shows current-light intensity (IL)
The calculation results of the characteristics are shown. Here, τ _n = τ _r = 3 ns
As a parameter, the photon lifetime τ _ph is 1 ps (solid line), 2 ps (dotted line), and 4 ps (dashed line). The size of the active layer is S _A = 400 μm ² , d = 0.
1 μm. The light intensity in FIG. 6C is the light intensity inside the semiconductor laser, and is larger than the light intensity emitted to the outside. Further, between the light intensity P and the photon number density S is related that _{P = hνυ g S B S ...} (10). Here, [nu the laser oscillation frequency are upsilon _g group velocity of the photon, the S _B is the beam cross-sectional area of the laser beam.

As can be seen from FIGS. 6A and 6C, when laser oscillation starts, the curvature of the IV characteristic changes.
As shown by the arrow in (a), the IV characteristics are slightly bent. The current value indicated by this arrow is the oscillation threshold. The state of the change of the curvature in the IV characteristic becomes clearer when the differential resistance dV / dI is considered as shown in FIG. In FIG. 6, the calculation is performed ignoring the effect of the contact resistance.

Next, an embodiment of the present invention will be described based on such a principle. FIG. 1 is a block diagram showing the configuration of an embodiment of the oscillation threshold value measuring device of the present invention. In FIG. 1, 1 is a current source, 2 is a protection resistor, 3 is a semiconductor laser to be measured, 4 is a digital multimeter, 5 is a personal computer, 6 is a laser beam emitted from the semiconductor laser 3, and 7 is an optical power meter. It is.

Here, in order to show that the oscillation threshold current I _th of the semiconductor laser 3 can be determined from the I-dV / dI characteristics, a semiconductor laser which emits laser light to the outside of the semiconductor laser is used. The -dV / dI characteristics are measured simultaneously. The semiconductor laser 3 is a distributed feedback laser (DFB-LD) having an oscillation wavelength of 1.55 μm, and is connected in series with the 50Ω protection resistor 2. When measuring the oscillation threshold value of the semiconductor laser 3, the personal computer 5 controls the programmable current source 1 to increase the current value in steps of 0.1 mA.

At the same time as increasing the current value, the voltage between the anode and the cathode of the semiconductor laser 3 is measured by the digital multimeter 4, the output from the digital multimeter 4 is input to the personal computer 5, and dV / dI is output from the personal computer. And outputs the I-dV / dI characteristic. The intensity of the laser beam 6 emitted from the semiconductor laser 3 is measured by an optical power meter 7, the output of the optical power meter 7 is input to a personal computer 5, and the IL characteristics are output using the personal computer.

FIG. 2 shows these I-dV / dI characteristics and I-dV / dI characteristics.
This is an output obtained by superimposing the L characteristic. In a real semiconductor laser, the effect of Schottky contact is included in the contact resistance. Also, since the current was changed in steps of 0.1 mA, the bending of the differential resistance dV / dI was not as clear as the calculation. However, when the normal direction of the paper surface is inclined from 70 degrees to nearly 80 degrees with respect to the line of sight and FIG. 2 is viewed from the side where the current value is larger, the position of the arrow in FIG. 2 (current I = 38 m
In A), the bending of the differential resistance dV / dI can be observed. Compared with the IL characteristic shown in FIG. 2, it can be seen that this current value I = 38 mA is the oscillation threshold.

In this case, the differential resistance is calculated using the personal computer 5, but the differential resistance is not limited to the differential resistance, and any parameter can be used as long as the change in the curvature of the current-voltage characteristic of the semiconductor laser 3 can be understood. It goes without saying that any calculation may be performed.

Further, nothing is provided with the personal computer 5
The differential resistance can be checked with a simple circuit without using. FIG. 3 shows the configuration of this measuring device. In the figure, 11 is a differentiating circuit, 12 is a differentiating circuit, and 21 is a dividing circuit. By this combination, the division circuit 21
DV / dI can be obtained from FIG. 4 shows an example of the differentiating circuits 11 and 12, and FIG. The differentiating circuits 11 and 12 include an amplifier A1, resistors R _s and R
_f , capacitors C _s and C _f . The dividing circuit 21 includes resistors R _{1 to} R ₆ and transistors Tr ₁ and T _r1 .
_r2, is composed of an amplifier A _2, A _3.

FIG. 7 is a diagram showing another embodiment of the present invention. In the present embodiment, the oscillation threshold of a ring laser composed of a semiconductor optical amplifier and an optical fiber is measured. As is well known, the ring laser has a configuration in which an optical fiber is added to a semiconductor laser (semiconductor optical amplifying element). Therefore, the principle of measuring the oscillation threshold is exactly the same as that of the semiconductor laser described in the above embodiment. . FIG.
In the figure, 100 is a semiconductor optical amplifying element, and 101 is a polarization maintaining fiber. As the semiconductor optical amplifying element 100, a traveling-wave amplifier having an antireflection film (residual reflectance of about 0.01%) formed on the end face of the Fabry-Perot laser is used. The center of amplification is at a wavelength of 1.55 μm.

Reference numeral 102 denotes a current source for supplying a current to the semiconductor optical amplifier 100; 103, a protection circuit; 104, a voltmeter for measuring the voltage between the anode and cathode of the semiconductor optical amplifier 100; A temperature control device 106 for controlling the temperature of the element 100 to a constant value is an optical connector. 107 is a personal computer that calculates the current-voltage characteristics, current-differential resistance characteristics, and current-light output characteristics of the ring laser, 108 is an optical coupler that extracts light from the ring laser, and 109 is the light amount extracted from the optical coupler 108. An optical power meter for measurement, 110 is an optical isolator, and 111 is an optical fiber.

When measuring the oscillation threshold of the ring laser, the current injected from the current source 102 into the semiconductor optical amplifier 100 is increased, and the voltage between the anode and cathode of the semiconductor optical amplifier 100 is measured by the voltmeter 104. measure.
The output of the voltmeter 104 and the amount of injected current of the current source 102 are input to the personal computer 107, and the personal computer 107 calculates dV / dI based on the output of the voltmeter and the amount of injected current. I-
dV / dI characteristic).

On the other hand, the light in the ring laser constituted by the semiconductor optical amplifier 100 and the optical fiber 101 is taken out to the outside by the optical coupler 108, and the light is measured by the optical power meter 109. Then, the measured value of the optical power meter 109 is transferred to the personal computer 10.
7, the personal computer 107 outputs current-light output characteristics (IL characteristics) based on the measured value of the optical power meter 109 and the amount of current injected from the current source 2.

FIG. 8 shows the I-dV / dI characteristics and the IL characteristics of the ring laser obtained in this manner. As apparent from FIG. 8, the injection current amount is about 11.5.
At mA, a large change appears in the differential resistance (the position of the broken line in FIG. 8). This is because good ohmic contact is realized in the present semiconductor optical amplifier. Further, from the IL characteristics shown in an overlapped manner, it is understood that the current value corresponding to the position indicated by the broken line in FIG. 8 is the oscillation threshold value.

FIG. 9 shows calculated values of current-voltage characteristics, current-differential resistance characteristics, and current-light output characteristics of the ring laser according to the present embodiment. Here, it is assumed that ohmic contact is realized, and the contact resistance is set to 4Ω. The calculation result is calculated as a graph based on the above-described principle using the apparatus shown in FIG. FIG. 9 (a)
9 shows current-voltage characteristics of the ring laser, FIG. 9B shows current-differential resistance characteristics, and FIG. 9C shows current-light output characteristics. In FIG. 9A, a change point of the curvature of the current-voltage characteristic is illustrated. Note that FIG. 9 performs calculations in the same numerical range as FIG. In FIG. 9, the photon lifetime is 4
The characteristics when ps, 3 ps, and 2 ps are set are shown. As is apparent from FIG. 9, the differential resistance sharply changes at the injection current amount corresponding to the oscillation threshold value clearly seen from the current-light output characteristics. That is, this means that the laser oscillation threshold can be obtained based on the current-differential resistance characteristics.

In the embodiment shown in FIG. 7, the differential resistance is calculated by using the personal computer 107. However, the present invention is not limited to the differential resistance and the curvature of the current-voltage characteristic of the semiconductor optical amplifying element 100 may be calculated. Any parameter may be calculated as long as it is a parameter that indicates the change in In addition, as described in the embodiment of FIG. 1, the check can be performed without using the personal computer 107 or using a circuit as shown in FIG.

[0041]

As described above, according to the present invention, the light intensity of the emitted light of the semiconductor laser is measured by measuring the oscillation threshold based on the change in the curvature of the current-voltage characteristic of the semiconductor laser. The oscillation threshold can be measured without the need to accurately measure the oscillation threshold even when the intensity of the laser light emitted to the outside of the semiconductor laser is low or the laser light is not emitted outside. .
Further, by measuring the oscillation threshold based on the change in the derivative of the current-voltage characteristic of the semiconductor laser, the oscillation threshold can be measured more accurately.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a semiconductor laser oscillation threshold value measuring apparatus according to the present invention.

FIG. 2 shows the I-dV of a semiconductor laser using the apparatus of FIG.
It is a figure showing the result of having measured / dI characteristic and IL characteristic.

FIG. 3 is a block diagram showing another embodiment of the present invention.

FIG. 4 is a circuit diagram illustrating an example of a differentiating circuit according to the embodiment of FIG. 3;

FIG. 5 is a circuit diagram illustrating an example of a division circuit according to the embodiment of FIG. 3;

FIG. 6 shows IV characteristics of a semiconductor laser, and I-dV / dI.
It is a figure showing the calculation result of a characteristic and IL characteristic.

FIG. 7 is a diagram showing another embodiment of the present invention.

8 is a diagram showing current-differential resistance characteristics and current-light output characteristics of the ring laser obtained in the embodiment of FIG. 7;

9 is a diagram illustrating current-voltage characteristics, current-differential resistance characteristics, and current-light output characteristics of a ring laser calculated in the embodiment of FIG. 7;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Current source 2 Protective resistor 3 Semiconductor laser 4 Digital multimeter 5 Personal computer 6 Laser light 7 Optical power meter 11, 12 Differentiation circuit 21 Division circuit 100 Semiconductor optical amplifier 101 Polarization holding fiber 102 Current source 103 Protection circuit 104 Voltmeter 105 Temperature control device 106 Optical connector 107 Personal computer 108 Optical coupler 109 Optical power meter 110 Optical isolator 111 Optical fiber

Claims (6)

[Claims] 1. A method for measuring an oscillation threshold value of a semiconductor laser, comprising: measuring an oscillation threshold value of the semiconductor laser based on a change in curvature of a current-voltage characteristic of the semiconductor laser. 2. A method for measuring an oscillation threshold value of a semiconductor laser, comprising measuring an oscillation threshold value of the semiconductor laser based on a change in a derivative of a current-voltage characteristic of the semiconductor laser. 3. An apparatus for measuring an oscillation threshold of a semiconductor laser, comprising: a circuit for calculating an oscillation threshold of the semiconductor laser based on a change in curvature of a current-voltage characteristic of the semiconductor laser. 4. An apparatus for measuring an oscillation threshold value of a semiconductor laser, comprising a circuit for calculating an oscillation threshold value of the semiconductor laser based on a change in a derivative of a current-voltage characteristic of the semiconductor laser. 5. A ring laser comprising a semiconductor optical amplifier and an optical fiber, wherein a laser oscillation threshold value is measured based on a change in a curvature or a derivative of a current-voltage characteristic of the semiconductor optical amplifier. Method for measuring the oscillation threshold of a semiconductor laser. 6. A ring laser comprising a semiconductor optical amplifying element and an optical fiber, comprising a circuit for calculating a laser oscillation threshold based on a change in a curvature or a derivative of a current-voltage characteristic of the semiconductor optical amplifying element. An apparatus for measuring an oscillation threshold value of a semiconductor laser, comprising: