JPS5928302B2 - Laser light transmission equipment - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a laser diode (LaserDiode).
The present invention relates to a laser beam transmission device that uses a LD (hereinafter referred to as LD) as a light source and optically transmits an analog signal through an optical fiber transmission line.

In conventional optical fiber transmission systems, when an LD is used to directly modulate an analog signal, a practically usable transmission quality cannot be obtained due to nonlinear distortion and noise of the LD.

Therefore, a light emitting diode (LightEmi) is used as a light source.
A pre-modulation method has been adopted in which the influence of non-linear distortion and noise of the LD is removed by using an LED (hereinafter referred to as LED) or by subjecting the analog signal to subcarrier modulation or pulse modulation in advance. A laser beam transmission device using this premodulation method will be explained below with reference to the drawings.

In FIG. 1, 10 is a premodulator that converts the analog input signal a into an FM modulated wave of a subcarrier or pulse modulation, -100 is an LD drive circuit connected to this premodulator 10, and 2 is LD driven by this LD drive circuit 1
, 4 is a photodiode that receives the optical signal output from the LD 2 and outputs an electric signal according to the optical signal, and 3 is an APC amplifier that amplifies the output signal of the photodiode 4 and adds it to the LD drive circuit 1. , The optical transmitter 12 is configured by the above. Moreover, 6 is an optical fiber cord whose one end is connected to the above-mentioned LD2. Furthermore, T is connected to the other end of the optical fiber cord 6, and is an APD (Avalanche Photo) that outputs an electrical signal according to the received optical signal.
toDiode), 8 is an amplifier for amplifying the output electrical signal of this APDT, 9 is connected to this amplifier 8, and the above A
The AGC amplifier 11 applies its output to the PD 7, and the demodulator 11 demodulates the output signal of the amplifier 8 and outputs an analog signal.The optical receiver 13 is constituted by the above. Next, the operation will be explained.

On the optical transmitter 12 side, the analog input signal a is sent to the premodulator 1
0, and this analog input signal is converted to a subcarrier, such as an FM modulated wave, or pulsed modulated. The output of the premodulator 10 is applied to the LD drive circuit 1, and the LD drive circuit 1 drives the LD 2 to output an optical signal according to the output of the premodulator 10. In this way, LD
The optical signal outputted by 2 is sent to an optical fiber cord 6 and guided to a photodiode 4. The optical signal guided to the photodiode 4 is converted into an electrical signal, and the electrical signal output is amplified by the APC amplifier 3 and fed back to the LD and drive circuit 1. In this way, the output optical signal of the LD 2 is controlled by automatic phase control. will be carried out. Above optical fiber cord 6
The optical signal sent to is transmitted by the optical fiber cord 6 and reaches the optical receiver 13. APD 7 receives this optical signal, converts it into an electrical signal, and applies this electrical signal to amplifier 8 . Amplifier 8 amplifies the signal and applies the amplified signal to demodulator 11 and AGC amplifier 9. The signal applied to AGC amplifier 9 is fed back to APD 7 by AGC amplifier 9, automatic gain control is performed, and the output level of amplifier 8 is kept constant. The demodulator 11 demodulates the original analog signal from the output signal of the amplifier 8 and outputs it. Conventional laser beam transmission equipment was configured as described above, and required a high-level modulation/demodulation circuit in addition to the optical modulation/demodulation circuit, which resulted in a complex configuration, an increase in the size of the equipment, and an increase in cost. There was a drawback that it was expensive.

Another drawback was that the distortion characteristics and delay characteristics of the premodulation circuit were problematic. This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and the LD
To provide a laser beam transmission device capable of directly optical AM modulating an analog signal without using a conventional premodulation circuit by adding an interference remover that removes nonlinearity and mode noise. The purpose is to

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 2, the symbols used in FIG. 1 indicate the same things as in FIG. Reference numeral 5 denotes an interference remover that is directly connected to the optical output end of the LD 2, and the portion surrounded by the one-dot chain line in the figure constitutes the optical transmitter 12''.

Further, 13' is an optical receiver consisting of an APD 7, an amplifier 8, and an AGC amplifier 9. Next, the operation will be explained.

The analog signal input a is applied to the LD drive circuit 1, and the LD drive circuit 1 drives the LD 2 to output an optical signal according to the analog signal input a. Photodiode 4
receives the output optical signal and converts it into an electrical signal,
This electrical signal is applied to the APC amplifier 3. The APC amplifier 3 amplifies this signal and feeds it back to the LD drive circuit 1, thus performing automatic phase control. The optical signal output from the LD 2 enters an interference remover 5 made of a rod glass directly connected to the optical output end of the LD 2.
The interference remover 5 removes nonlinear distortion and mode noise contained in the signal, and the signal is sent to the optical fiber cord 6. Note that the interference remover 5 is directly connected to the optical output end of the LD 2 as described above, so that mode noise etc. are not generated at the connection point between the interference remover 5 and the optical fiber cord 6, etc. When the transmitted optical signal is transmitted by the optical fiber cord 6 and reaches the optical receiver 13', the APD 7 receives the optical signal and applies it to the amplifier 8. The amplifier 8 amplifies the signal, and the amplified signal is fed back to the APD 7 via the AGC amplifier 9, thereby outputting an analog signal b subjected to automatic gain control. Next, the operating principle of the interference remover 5 will be explained. Nonlinear distortion and mode noise contained in the output of the LD are caused by the characteristics of the optical signal propagating within the optical fiber code.

In other words, it has been found that this is directly related to the speckle pattern in the optical power distribution in the cross section of the optical fiber cord.
This speckle pattern is caused by the coherence characteristic of laser light, that is, by the unity of the spectral mode of laser light. Therefore, in this embodiment, the output light of the LD is input to the interference remover at a stage before the output light enters the optical fiber cord, thereby disturbing the single spectral mode of the laser light and The spectral mode is expanded to eliminate the coherence of the laser beam. In this way, it is possible to prevent the generation of mode noise and nonlinear distortion as described above. Now, if we consider the case where the output light of the LD is transmitted through a rod glass in the air that has an aperture approximately the same as that of the optical fiber cord, the length l of the rod glass at which light coherence disappears and a speckle pattern does not occur is It will look like this:

8 IL Where is 1c-? ; LD coherence A foot length in air n1; refractive index NA of rod glass: numerical aperture Δλ of rod glass; spectral width (wavelength) of LD λ; spectral center wavelength of LD. So, for example, λ-0.85 μM, Δλ=18 Then, from the above formula! c'-2.2cm, and if n1=1.5, then IL-1.6cr1L.

Therefore, if this type of rod glass is used as an interference remover, the effects of mode noise etc. can be removed. In the above embodiment, a rod glass was used as the interference remover, but it is also possible to use an interference remover using various optical components such as lenses, interference films, mirrors, etc. that perform the same functions. The same effect can be obtained.

As described above, according to the present invention, the LD is connected to the output end of the LD.
By adding an interference remover that removes nonlinear distortion and mode noise contained in the , a laser beam transmission device that can directly modulate analog signals without using a conventional premodulation circuit can be obtained. There is.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional laser beam transmission device, and FIG. 2 is a block diagram of a laser beam transmission device according to an embodiment of the present invention. 2... Laser diode, 5... Interference remover, 12'... Optical transmitter, 6...
Optical fiber cord (optical fiber transmission line), 13'...
...Optical receiver.

Claims (1)

[Claims] 1. An optical system equipped with a laser diode that inputs an analog signal and outputs an optical signal, and an interference remover made of a rod glass connected to the laser diode and having a length that does not cause optical coherence of the output optical signal of the laser diode. A transmitter, an optical fiber transmission line for transmitting the output optical signal of the optical transmitter, and an optical receiver for receiving the optical signal transmitted by the optical fiber transmission line and outputting the analog signal. A laser beam transmission device featuring: