JPS594238A - Optical transmitter of analog signal - Google Patents

Abstract

PURPOSE:To transmit stably a signal with high quality at long range transmission, by applying amplitude modulation to an input signal and applying the result to a laser diode (LD) biased near a threshold current level so as to return the reflecting light from a connector during non-irradiating period of the LD. CONSTITUTION:An output of a high frequency oscillator 3 with variable oscillating frequency is used as a carrier of an amplitude modulator 2. An output current of a bias current generating circuit 4 is selected to a value close to the threshold current level of the laser diode (LD) 6. An irradiated light from the LD6 is coupled and supplied to a fiber 9 for transmission with a pig tail fiber 7 and a connector 8. Let the length of the pig tail fiber 7 be L, the frequency of carrier be fc, the period be tauc, and the refractive index of the fiber be (n), then the time difference tau1 between the irradiated light of the LD and the reflecting light from the connector 8 is tau1=2nL/C-m.tauc (where; C is the velocity of light and m is an integral number), since the reflecting light is returned during the non-illuminating period of the LD at tau1=tauc/2, the longitudinal mode of the LD is not almost subject to the effect of the reflecting light. Since the L is not always constant, the frequency of the carrier is adjusted to obtain tau1=tauc/2.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an analog signal optical transmission device that uses a laser diode as a light source to transmit all analog signals.

Conventional configurations and their problems Conventionally, when transmitting analog signals, light emitting diodes (hereinafter abbreviated as LEDs) have been used, which are easy to handle and highly reliable.
is often used as a light source. Unfortunately, long-distance transmission is not possible.

On the other hand, when a laser diode (hereinafter abbreviated as LD) is used as a light source, the coupling efficiency between the LD and the fiber is high, and the LD itself has good linearity, so long-distance transmission is highly likely to be realized. However, when actually constructing a transmission system using an LD as a light source, the characteristics of the transmitted signal may deteriorate significantly due to the generation of speckle noise (modal noise) within the fiber due to the good coherence of the LD. Are known.

As a countermeasure against this speckle noise, it is effective to multiply the longitudinal modes of the LD.
The use of , high frequency superposition method, etc. are adopted.

Among these methods, the longitudinal mode multi-LD has problems in terms of manufacturing method, reliability, and linearity, and is generally difficult to obtain. On the other hand, the high-frequency superposition method uses high-frequency pulses to superimpose the transmission signal to
(Z multimode method. In this method, the transmission signal is chopped with high frequency pulses,
The current band that can be applied to the LD requires the baseband signal band of the transmission signal and the band of the high frequency pulse to be applied, and a wide band circuit from low frequency to high frequency is required. Moreover, this method alone has no effect on the variation in the characteristics of the LD due to the reflected reinjected light into the LD. The reflected reinjected light to the LD is mainly from the fiber end face, and is due to Fresnel reflection from the connector end face. This reflected reinjected light to the LD causes reflection noise in the LD or fluctuations in the LD's oscillation mode. These not only increase the noise of the LD, but also cause deterioration of linearity. Furthermore, it is very susceptible to mechanical fluctuations in the fiber (eg, bending, twisting, etc.), temperature changes, changes in the bias current of the LD, and the like. Therefore, the influence of these reflected lights cannot be suppressed only by high frequency superimposition.

Purpose of the Invention As mentioned above, the present invention aims to suppress speckle noise that becomes a problem when transmitting all analog signals using an LD as a light source, and simultaneously suppresses fluctuations in the longitudinal mode of the LD due to reflected reinjected light. The present invention provides an optical transmission device that can stably transmit high-quality signals during long-distance transmission.

Composition of the Invention The present invention drives a laser diode by superimposing a high frequency on an analog signal, and also allows reflected light from a connector closest to the laser diode to return to the laser diode during the non-emission period of the laser diode. It is designed to have good characteristics and stable transmission.

Description of examples Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, an input signal 1 is first amplitude modulated by an amplitude modulator 2. In FIG. As the carrier wave of the amplitude modulator 2, the output of a high frequency oscillator 3 whose oscillation frequency is variable is used. The output of the amplitude modulator 2 is added to the output of the bias current generation circuit 4 by an adder circuit 6, and the sum is supplied to the LDe. The output current of the bias current generation circuit 4 is set to a value close to the threshold current of the LDe.

The light emitted from the LDe is coupled to a fiber 9 by a pig-tilt fiber 7, and then transmitted.

One end of the pigtil fiber 7 is normally connected to a transmission fiber 9 by an optical connector 8 . The connector closest to LD6 is usually this connector 8, and the LD
It is assumed that the length of the fiber between E and the connector 8 closest to LDe is the same as the length of the pig-til fiber 7.

The relationship between the current waveform of the LD and the optical output waveform at this time will be explained using FIG. 2. FIG. 2 is a so-called I-L characteristic, which shows the current versus light output characteristic of the LD. Here, 10 is its characteristic line. The lower part of FIG. 2 shows the LD current according to the present invention. 11 is a modulation current waveform, and ■th is a threshold current. The optical output waveform at this time is shown on the right side of FIG. 12 is an optical output waveform, which is a waveform obtained by half-wave rectification of the modulation signal by 7 degrees. In order to make the longitudinal mode of the LD multi-mode, the carrier frequency is preferably 100 MHz or more. Therefore, the high frequency oscillator 3 whose oscillation frequency is variable is at least 100
It must be possible to oscillate at MHz or higher.

By the way, now, assuming that the length of the pig-tilt fiber 7 is L (see FIG. 1), what happens to the reflected light from this connector end face will be explained using FIG. 3. In FIG. 3, the horizontal axis is time and the vertical axis is optical power. Hatching part 1 in Fig. 3
6 is the light emission waveform of the LD, and a broken line waveform 16 shows the reflected light from the connector. The frequency of the carrier wave is f. 9 periods is τ.

If (fc=1/τo), the light emission period of L'D is τ. Let n be the refractive index of a 0 fiber, then the time it takes for it to exit the LD, reflect at the end face of the fiber, and return to the LD again is τ1, then τ -2nL/ (c; speed of light)...
...(1) becomes 0. At this time, the time difference τ1 between the emitted light and the reflected light of the LD is τ1-τr-m·τ. (m; integer)・・・・・・・・・
(2) LL when τ1-Q) When the emitted light is maximum, the reflected light is also maximum. At this time, the longitudinal mode of the LD is most affected by the reflected light. When a G i (graded index) fiber is used as the pig-til fiber 7, and the above conditions are satisfied, if the fiber is slightly bent or twisted, the longitudinal mode of the LD becomes almost completely single mode, or becomes considerably There are several multi-mode modes.

Such fluctuations in the longitudinal mode of the LD are caused by coherent interference between the emitted light and the reflected light from the LD, and are caused not only by mechanical fluctuations such as bending of the fiber, but also by changes in the LD's oscillation wavelength (bias current and It also occurs due to minute changes in the refractive index of the fiber due to temperature fluctuations) or due to temperature changes in the refractive index of the fiber.

Therefore, when transmitted over long distances under such conditions,
Speckle noise increases when the LD longitudinal mode is single mode, and decreases when it is multimode.
The S/N ratio will vary widely over time.

Further, fluctuations in the longitudinal mode of the LD not only cause fluctuations in noise but also impair linearity. Therefore, for example, NTSC
When transmitting a video signal using this method, the DG and DP characteristics may deteriorate.

Next, when τ1-rc/2, the reflected light returns during the non-emission period of the LD, so the longitudinal mode of the LD is almost unaffected by the reflected light, and the fiber cannot be bent. However, the longitudinal mode of the LD does not change and remains stable. Therefore, under these conditions, the LD is capable of stable multimode oscillation, so the S/N of the transmission signal is good and stable, and the linearity, DG, and DP characteristics are also good.

By the way, the length of the pig-tilt fiber attached to the LD is not necessarily constant. Therefore, by adjusting the frequency of the high-frequency oscillator whose oscillation frequency is variable according to the length of the pig-til fiber, that is, the distance between the LD and the connector closest to the LD, so that τ1 = "cA",
The longitudinal mode of the LD is hardly affected by reflected light.

In order to achieve τ1-rC/2, it is possible not only to adjust the carrier frequency but also to adjust the length of the pig-tilt fiber. When τ1=rC/2 is used using equations (1) and (2), the following is obtained. That is,
Carrier wave oscillation period τ. In the equation, the length of the fiber from the LD to the connector end closest to the LD is the distance (- τ) times the distance traveled by light (velocity; c/n) traveling through the fiber (m 10μ). As a result, the reflected light from the connector returns to the LD when the emitted light is zero. In this case, the oscillation frequency may be fixed, so the circuit section is simple.

In addition to the connectors considered here, there are coils and lids in the optical transmission path, but they do not cause any major problems because the reflected light loses its coherent toy when transmitted over long distances. In fact, the non-emission period of the LD and the return time of the reflected light do not need to be exactly the same, it is sufficient that they coincide.

The experimental result, τ, is τ. Approximately ±0.3 centered on /2
(rC/2) Even in the range of By applying this voltage and allowing the reflected light from the connector to return during the non-emission period of the LD, the longitudinal mode of the LD becomes a multi-mode and becomes stable. No, the speckle noise can be suppressed,
In addition, variations in the beam due to temperature changes or fiber bending do not occur. Also, when transmitting a video signal, DG and DP
The characteristics also become better, and temporal fluctuations are also reduced.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an analog signal optical transmission device in an embodiment of the present invention, and Fig. 2 is a block diagram of an LD used in Fig. 1.
FIG. 3 is a diagram showing the time relationship between LD emitted light and reflected light. 1... Input signal, 2... Amplitude modulator,
3...High frequency W unit, 4...Bias current generation circuit, 5...Addition circuit, 6...
Laser diode, 7...Pig till fiber, 8...Connector, 9...Transmission fiber. Name of agent: Patent attorney Toshio Nakao and 1 other person 2nd
Figure 3

Claims (2)

[Claims] (1) A laser diode is used as a light source, and a high frequency is superimposed on an analog signal to drive the laser diode, and during the non-emission period of the laser diode, the reflected light from the connector closest to the laser diode is transmitted to the laser diode. An analog signal optical transmission device characterized in that it is configured to return to. (2) A high-frequency oscillator with at least a variable oscillation frequency, which drives the laser diode by superimposing high-frequency waves;
The invention is characterized by comprising an amplitude modulator that amplitude modulates an input signal using the output of the oscillator as a carrier wave, and an addition circuit that adds the output of the amplitude modulator to the laser diode biased near a threshold current. An analog signal optical transmission device according to claim 1. an amplitude modulator that amplitude modulates an input signal using the output of the oscillator as a carrier wave; and an adder circuit that adds the output of the amplitude modulator to a laser diode biased near a threshold current; The fiber length between the connector end faces closest to the laser diode of and is set so that the reflected light from the connector end face returns to the laser diode during the non-emission period of the laser diode. 2. The analog signal optical transmission device according to item 1.