JPS5994482A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To facilitate the adjustment in a laser element drive by setting a voltage of one signal which is inputted to a non-inverting terminal of a comparator by bringing the DC bias current transmitted by the drive unit into coincidence with the value at the modulation high level time. CONSTITUTION:Modulation level signals 0, 1 from a modulator 4 are fed in response to the presence or absence of modulation to the non-inverting input terminal of a comparator 5, divided by a variable resistor VR2 and a resistor R and injected. The comparator 5 produces a compared difference output of the detection signal from the detector 3 to the drive unit 2 with the injected input as a control signal. When the modulation input a is high level and the modulation level signal is 0, the set values of the resistors VR2 and the resistor R can be ignored. Then, the modulation input is as low level and the modulation level signal is as 1. Here, the signal 1 is set by the resistor VR2 so that the DC bias current obtained when the DC bias current of the drive unit 3 is high level becomes coincident to the DC bias current value at the modulation signal current superposing time. In this manner, the setting of the resistor VR2 does not exert the influence to the adjustment at the high level time.

Description

[Detailed description of the invention] (a) Technical field of the invention The present invention relates to improvements in semiconductor laser drive circuits.

(b) Background of the technology Semiconductor laser devices are usually made by polishing or cleaving two parallel planes perpendicular to the junction surface of a compound semiconductor with a double heterostructure to create light-reflecting surfaces, and applying a bias current to both sides of the device. This is a 1st current-to-light conversion element that amplifies and oscillates photon radiation due to electron-hole recombination energy in the active layer of the resulting junction and outputs coherent light from the reflective surface. Most of the energy passes through the p and n regions and is attenuated by absorption coefficients specific to these regions, but in addition to this mode loss, there are transmission losses from the reflective surfaces on both end faces, and this loss is exceeded by the laser. There is a condition for starting oscillation, ie a threshold of bias current. The threshold current density in a laser semiconductor is over 150 A/d, and the luminous efficiency is low, so it is driven by a DC bias.Most of the DC is consumed as heat, so the cooling conditions for the device are severe, and the threshold voltage between the devices is low. The dependence of the current force variation and the aforementioned 11rt flow characteristics on the junction temperature is also extremely large.

(e) Conventional technology and problems Conventionally, semiconductor laser devices have a threshold value of
Since the current variation and the temperature dependence of the characteristics are large, there is a means to control the DC bias current according to the detection current of the light receiving element that receives light from the rear monitor source, which is proportional to the laser light output on the side opposite to the laser light output side of the element. There are few drive systems using a unique DC bias current based on open control. Detection of conventional monitor light 'Due to the drive method using feedback control based on direct current, semiconductor laser elements have many variations in characteristics and often produce invisible light.
The means for setting the optical output to a specified value is based on a trial-and-error method of determining Nu based on measurement data, which has the drawbacks of being troublesome and increasing costs.

(d) Object of the Invention An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a semiconductor laser drive circuit that can easily and efficiently adjust the driving pitch of a semiconductor laser.

(e) Structure of the Invention The object of the present invention is to provide a drive section that sends out a DC bias current for a semiconductor laser device, a detection section that receives and detects the rear monitor light of the semi-integrated laser device and sends out a detection current, and a drive section that sends out a DC bias current of a semiconductor laser device, a detection section that receives and detects the rear monitor light of the semi-integrated laser device and sends out a detection current, and a drive section that sends out a DC bias current of a semiconductor laser device. A modulation section that sends out a modulation signal to be superimposed on the DC bias current and its modulation level signal, and a differential signal of the detected current and a set current from a separate reference voltage source and a differential level signal are input and compared. It is equipped with a comparison section that sends the value as a control signal to the drive section, and during modulation, the uOpy signal corresponding to the variable rate high level is input from the modulation section to the non-inverting terminal of the comparison section, and the uOpy signal corresponding to the variable high level is input to the inversion terminal from the detection section. When inputting the detection current and obtaining a modulation high-level control signal in the comparator section, detection is performed so that the optical output of the semiconductor laser due to the superposition of the DC bias current caused by the control signal and the modulation signal becomes a preset value. In addition to setting the current generated by the reference voltage source to be added to the current, when the modulation level is low, an addition signal of the detected current and the current generated by the reference voltage source is input to the inverting terminal, and the modulation unit sends it to the non-inverting terminal. When inputting a tlljj signal corresponding to a low level to obtain a modulation low level control signal in the comparison section, the DC bias current sent out by the drive section is input to the non-inverting terminal so that it matches the value at the time of the modulation high level. This can be achieved by providing a semiconductor laser drive circuit characterized by driving the voltage of the "'1" signal.

(f) Embodiment of the Invention An embodiment of the invention will be described below with reference to the drawings. FIG. 1 shows a semiconductor laser drive circuit according to an embodiment of the present invention, and FIG. 2 shows a concrete example block diagram of its modulation section. In the figure, 1 is a semiconductor laser element, 2 is a drive section, 3 is a detection section, 4 is a modulation section, 5 is a comparison section, R is a resistor, VRI, VB2 is a variable resistor, ■ is a constant current circuit,
Qo, Qt are NP N, ) transistors and RL are load resistances.

When the modulation level is low, the semiconductor laser element 1 receives a DC bias current from the drive unit 2, and when the modulation level is high, it is superimposed on the DC bias current and the modulation input is applied to the modulation unit 4. A light emitting operation is performed by applying a modulation signal sent out according to the following. The drive section 2 sends out a DC bias current according to the control signal sent from the comparison section 5. The detection section 3 uses a light receiving element to detect the light/current conversion of the rear monitor light which is proportional to the output light of the semiconductor laser element, and sends it to the comparison section 5 as a detection signal. The modulator 4 is configured, for example, as shown in the specific example block diagram of FIG.

Transistors Q+ and Qs have a switching function using an emitter-coupled logic circuit, and when the modulation input signal exceeds a threshold voltage Vf, which is preset between the high and low levels of the modulation input signal, transistor Q1 is turned on.
When Q is off and in low phase, transistor Q4 turns off and Q turns on, so transistor Q
When a high level exceeding the threshold value Vf is detected as a modulation level signal to the comparator 5 that sends out the collector potential of 2, a ``0'' signal, that is, the ground level is sent, and when it is a low level, a 1
``signal, that is, the collector potential of the transistor Q2 determined by the current source ■ and the load resistor RL.Next, the comparator 5 sends out the detection signal and current of the detector 3 and a separate reference signal to its inverting input terminal, for example. The reference voltage Vref of the voltage source is injected with a current set by the variable resistor VR1, and the modulation level signal tl from the modulation unit 4 is injected into the non-inverting input terminal.
O'''1〃 is sent out depending on the presence or absence of modulation, and is divided by variable resistor VR2 and resistor R and injected. Comparator 5
sends the comparison difference output to the drive section 2 as a control signal. Here, the modulation input is set to high level and the modulation level signal is set to O.
”, the ground/α position is given to the inverting input terminal of the comparator 50, and the set values of the variable resistor VR2 and the resistor R can be ignored.At this time, the DC bias current of the drive unit 2 and the modulation signal current The current, that is, the addition current (is output. At this time, the optical output of the semiconductor laser element 1 is separately measured, and the variable resistor VR1 is adjusted so as to obtain a control signal for the drive section 2, that is, the comparator 5, so that the optical output of the semiconductor laser element 1 becomes the set value. Adjust the value and set the addition current from the reference voltage Vref that should be applied to the detection signal ML of the detection section 3.Next, the modulation input is set to a low level and the modulation level signal is set to 1''.In this case, @ A detection signal driven by a DC bias current on which the modulation signal '1 stream is not superimposed is applied from the detection unit 3. Here, the drive unit 30
The DC bias current is controlled by the control signal to a value equal to the DC bias current obtained when the above-mentioned modulation level is high and the DC bias current measured in advance when the modulation signal current is superimposed. If the modulation level signal input to the non-inverting terminal is set to 11" by the variable resistor VR2, the ground level is output as the modulation level signal '0' at the recitation level, so as mentioned above, The setting of variable resistor VR2 has almost no effect on the adjustment at high level, and the setting by variable resistor VR at high level is the optical output of high level vs. low level according to the modulation signal = m. If the ratio is large, the variable resistor VR will not reach the low level.

The influence of the addition 'i current by the reference voltage Vref due to the setting of is small, and the driving circuit of the semiconductor laser element 1 can be set to a predetermined value by adjusting once for each level.

(g) As described in detail, according to the present invention, it is possible to eliminate variations in the characteristics of a semiconductor laser device, especially high and low levels during modulation, without checking the threshold current of laser oscillation. Since they can be set independently without affecting each other, adjustment in driving the semiconductor laser element is easy, and since the direct current flowing at high and low levels of the modulation signal is equal, the data mark rate can be improved. This is useful because a circuit with no fluctuation in optical output can be obtained.

[Brief explanation of the drawing]

FIG. 1 shows a semiconductor laser driver according to an embodiment of the present invention. A block diagram of the auxiliary circuit and FIG. 2 are specific example block diagrams of the modulation section. In the figure, 1 is a semiconductor laser confectionery, 2 is a drive unit, and 3 is a semiconductor laser confectionery.
is a detection section, 4: d modulation section, 5 is a comparison section and VR, , V
It is an R2h4 variable resistor.

Claims (1)

[Claims] A drive unit that sends out a DC bias current of a semiconductor laser element, a detection unit that receives and detects the rear monitor light of the semiconductor laser element and sends out a detection current, a modulation signal that is superimposed on the DC bias current according to an input modulation signal, and its modulation. A modulation section that sends out a level signal, a ratio [section] that differentially inputs the sum signal of the detected current and a set current from a separate reference voltage source, and the modulation level signal, and sends the comparison value as a control signal to the drive section. 9. During modulation, the non-inverting terminal of the comparison section is supplied with 0" corresponding to the modulation high level from the modulation section.
By inputting the signal and the detection current from the detection unit to the inverting terminal to obtain a control signal with a high level of modulation in the comparison unit, the DC bias current caused by the control signal and the modulation signal are superimposed, and the semiconductor In order to make the output light of the laser become a preset value, the current by the reference voltage source to be added to the detection current is set, and when the modulation level is low, the detection current and the current by the reference voltage source are added to the inverting terminal. 0 1 j) When inputting a signal corresponding to the modulation low level sent from the modulation section to the non-inverting terminal and obtaining the control signal of the transversal low level in the comparison section, the signal is sent to the drive section. A semiconductor laser drive circuit characterized in that the voltage of the u 1 n signal input to the non-inverting terminal is set so that the DC bias current matches the value at the time of the modulation high level.