JPS5928078B2 - Laser diode drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a drive circuit having functions of stabilizing the optical output of a laser diode and compensating for distortion.

A conventional drive circuit of this type is shown in FIG.

In the figure, 1 is a transistor, 2 is a transistor, 3
is a resistor, 4 is a resistor, 5 is a bias setting resistor for the base of transistor 1, 6 is a laser diode, T is a signal input terminal, 8 is a resistor, 9 is a coupling capacitor, 10 is one of the optical outputs from laser diode 6 This is a photodiode that receives light. Next, the operation will be explained.

A part of the optical output from the laser diode 6 is received by the photodiode 10 and current feedback is applied to the base of the transistor 1. Of the optical output from the laser diode 6, the DC optical component is used to stabilize the optical output. The distortion is compensated using the alternating current (signal) light component. When calculating the DC light component, if the light output PL is set at a certain temperature To and the temperature changes by △△T from To, the change in light output is calculated by Δ
If PL is the rate of change in optical output, then ΔPLΔTd
It is represented by (1) PLβIR-IthdT.

However, β is the DC current amplification factor β1 and β of transistor 1 and transistor 2. is the product of (β=β,・β.), and I
R is the current flowing through the resistor 5, Ith is the threshold current of the laser diode, and dlth/dT is the temperature change rate of Ith. From equation (1), it can be seen that even if Ith and β fluctuate due to temperature changes, if β1R>1th, the optical output PL is stabilized.

Next, calculations are performed on the AC light component.

The relationship between the signal Vs from the input terminal T and the optical output signal Ps from the laser diode 6 is determined using the circuit diagram shown in FIG.

The base input signal current of transistor 1 is ib, the current amplification factors of transistor 1 and transistor 2 are respectively β,
, β2, and the fractional quantum efficiency of the Lage diode 6 is nL.
Then ▲ v Tameko LJrlr Oto Tomb V

~two! becomes.

When a part of Ps αPs (α<1) is received by the photodiode 10 with sensitivity Np (A/W), the photodiode 10
The signal current 1p flowing in is as follows.

Base input signal current 1b is the sum of currents due to Ip and signal Vs. If the input impedance of the Darlington transistor circuit is Zi, then the following equation is obtained. ▲VO@! Formulas (2) to (4) are obtained.

Rg〉Zi eight eight If you choose becomes.

The above equation is a well-known equation representing the input/output relationship of negative feedback amplifiers in electronic circuits, and αNp represents the feedback ratio. Distortion can be suppressed.

However, for a Darlington-connected transistor, the 3d0 cutoff frequency Fc is expressed as '-11-t.

FT is the gain-bandwidth product. Therefore, when a transistor with a small FT is used or when used under conditions where β1 and β2 are large, Fc becomes extremely small, making it impossible to drive the laser diode 6 with a high frequency signal. Since the conventional laser diode drive circuit is constructed as described above, it has the disadvantage that the laser diode cannot be driven by a high frequency signal.

This invention was made to eliminate the drawbacks of the conventional ones as described above. By lowering the current amplification factor for AC signals of Darlington-connected transistors, the cutoff frequency was increased by 3 dB. The purpose is to provide a drive circuit that can drive a diode.

An embodiment of the present invention will be described below with reference to the drawings. Second
In the figures, the same reference numerals as in FIG. 1 indicate the same or equivalent parts. 11 is a capacitor, and 12 is a resistor.

Feedback is provided by a capacitor 11 and a resistor 12. Further, 13 is a diode. Next, the operation will be explained.

In FIG. 2, a feedback circuit including a capacitor 11 and a resistor 12 applies feedback to an AC signal, but since the DC component is blocked by the capacitor 11, no feedback is applied.

Therefore, the direct current amplification factor is not affected by the addition of the capacitor 11 and the resistor 12, and the optical output expressed by equation (1) is stabilized. On the other hand, the current amplification factor βf for an AC signal can be approximated by NI, where the value of the resistor 4 is Rc and the value of the resistor 12 is Rf.

For this reason, Rc and Rf can be selected appropriately, and the current amplification factor βf for the AC signal can be changed to the current amplification factor when there is no feedback (β
1.β2). For this reason, 3
The dB cutoff frequency Fc is as follows, and by applying feedback, it becomes possible to widen the band of the drive circuit. Next, the function of the diode 13 is to prevent damage to the photodiode 10 due to forward current flowing through the photodiode 10 when the power is turned off.

In other words, considering the case where there is no diode 13, the capacitor terminal connected to resistor 12 is connected to resistor 4 due to the bias conditions of transistors 1 and 2 during circuit operation.
The potential is lower than that of the terminal connected to. Therefore, when the power is turned off, the capacitor 11 discharges the stored charge, but in this case, the discharge current flows from the photodiode 10 to the resistor 12, so there is no forward current for the photodiode 12. becomes. If the capacitance of the capacitor 11 is large, this forward current will become larger than the rated current of the photodiode 10 and damage the photodiode 10. If the diode 13 is connected, such a forward current for the photodiode 10 becomes a reverse current for the diode 13, so this forward current will no longer flow and the photodiode 10 will not be damaged. During circuit operation, a reverse current flows through the photodiode 10, and since this current is a forward current with respect to the diode 13, the connection of the diode 13 has no effect. In the above embodiment, the case where two NPN transistors are connected in Darlington is explained, but any number of transistors may be used.
In addition to NPN transistors, PNP transistors are also possible by connecting the ground terminal to the positive power source and grounding the negative power source as shown in FIG.

As described above, according to the present invention, since feedback is applied to the Darlington-connected transistors,
This has the effect of compensating for distortion in a broadband laser diode that has an optical output stabilization function.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a conventional laser diode drive circuit, and FIG. 2 is a block diagram showing a laser diode drive circuit according to an embodiment of the present invention.

Claims (1)

[Claims] 1. An input terminal for inputting a drive signal, a Darlington-connected amplifier circuit consisting of a plurality of transistors for amplifying the signal from this input terminal, and an amplifier circuit connected to the collector circuit of the final stage transistor of this amplifier circuit for inputting the signal from the input terminal. a laser diode that emits light in response to a signal; a photodiode that receives a portion of the light emitted signal from the laser diode to control the base bias of the first stage transistor of the amplifier circuit; A laser diode drive circuit consisting of a feedback circuit consisting of a series circuit of a resistor and a capacitor connected to the base circuit of a transistor.