JPS6120913A - Optical transmitter of laser diode - Google Patents

Abstract

PURPOSE:To suppress the noise to transmit signals of high quality by leading the output light of a laser diode LD to a multimode fiber, whose end face is inclined, through a single mode fiber whose one end is a vertical end face and is coated with a reflecting mirror. CONSTITUTION:The laser beam from an active layer 101 of an LD1 placed on a stem 2 is led to a core 102 of a short single mode pigtail fiber 3, whose one end is worked to a taper ending ball and the other end is coated with a mirror 4 having a prescribed reflection factor, through a taper ending ball 103. The light emitted from the mirror end 4 of the fiber 3 is led to a core 10 of a multimode fiber 5 for transmission, whose end face is inclined, and is transmitted. Thus, the wavelength shift of the LD due to LD direct current modulation is suppressed to suppress the signal distortion for multimode fiber transmission, and the LD noise is oscillated in the single mode to reduce the LD noise, and the modal noise is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to optical communication using an LD (laser diode) as a light source, and more particularly to a device that performs optical transmission by directly modulating the intensity of an LD with an analog signal current.

Conventional configuration and its problems The method of transmitting light using a multi-mode fiber using an LD as a light source was expected to be a good analog signal transmission system due to the good linearity of the LD and the broadband properties of the fiber. In reality, due to the occurrence of modal noise and the phenomenon of increased noise of the LD itself due to reflected reinjected light to the LD,
At present, it is not put into practical use. Furthermore, it is assumed that the main cause of modal noise is the good coherence of the LD, and methods such as high-frequency superposition, optical feedback, multi-mode light source, and broadening of the spectral linewidth have been proposed and studied as mitigation measures. ing,. However, this method has the disadvantage of increasing noise due to spectrum broadening, and although it is possible to transmit video signals of approximately 1 ah, multiple ah (e.g. frequency multiplexing)
FDM) signals cannot be sent.

Purpose of the Invention The present invention has been made for the purpose of suppressing the generation of modal noise and reflection noise due to the use of an LD and a multimode fiber using a simple method without making the LD multimode, as described in the conventional example. It is something that

Structure of the Invention The present invention uses an LD as a light source, and guides its output light into a multi-mode fiber through a single-mode fiber.One end of the single-mode fiber is coated with a reflection coating on a vertical end surface, and a multi-mode light source is guided into a multi-mode fiber opposite to the vertical end surface. The end face of the mode fiber is an oblique end face to suppress the generation of modal noise and noise.

DESCRIPTION OF EMBODIMENTS The basics of the present invention are to unify the spectrum of the LD by adding an external resonator to the LD, and to reduce the L when directly modulating the current.
This takes advantage of the fact that the D oscillation frequency shift can be suppressed. (For details, refer to IEICE Light Quantity Elm Study Group Material QQE83゜76 "Noise and Modulation Characteristics of Optical Feedback Semiconductor Laser") The basic configuration of the present invention is shown in FIG. LDl is stem 2
The laser beam from the active layer 1o1 of the LDl is transmitted through a short single-mode Bingtil fiber 3 whose one end has a Taber tip and whose other end is coated with a mirror 4 with a reflectance R. It is guided to the core 1' 02 via the tapered ball 103. The light emitted from the mirror-coated end 4 of the short fiber 3 is guided to the core 104 of the transmission multimode fiber 6, which has an oblique end face, and is transmitted. In such a configuration, the characteristics when the LD is modulated in brightness by current will be described below.

Now, the configuration shown in FIG. 1 is simplified and shown in FIG. 2. 1 is the LD, 4 is the reflection mirror, and 101 is the LD active layer.As shown in the figure, the LD length is 41, the active layer refractive index is n2, the effective length of the reflection mirror and LD is L, the amplitude reflectance of both end faces of LD is r1. r2. Let the amplitude reflectance of the reflecting mirror be ro. In such a case, if the wavelength shift of the LD due to modulation when there is no reflecting mirror 4 is Δλ', and the wavelength shift when there is a mirror is Δλ, then the degree of wavelength shift suppression due to the presence or absence of the mirror is Δλ
The relationship between /Δλ' and L/rJ is shown in FIG. 3 using ro(R-r'O) as a parameter. From this characteristic, L/n7
) 6° and r. >0.2, the wavelength shift suppression degree is 0.2.
Becomes 6 or less. In other words, the oscillation wavelength shift of the LD per unit current is less than %. Normally, the LD length l is about o,3fi, and the refractive index is about n+3.5, so nl is about lf
It becomes f1m.

The wavelength shift due to modulation is closely related to the amount of signal distortion after multimode transmission, and the smaller the amount of shift, the less deterioration of distortion. Therefore, the longer the external cavity length (the optical distance between the LD and the mirror), the greater the degree of shift suppression and the less deterioration of distortion. However, if the external cavity length is increased, L
The modulation degree at which D can maintain single mode oscillation becomes smaller.

Therefore, the external cavity length is 6flF~30
'Crn level is appropriate. The optimum cavity length varies depending on the application, ie, modulation frequency and modulation degree, but if it is less than 6 m, the degree of wavelength shift suppression will be small, and if it is more than 30 cm, the modulation degree that allows the LD to oscillate in a single mode will be extremely small. Therefore, this range is appropriate.

By the way, since the refractive index of the fiber is 1.5, the length of the pig-til fiber 3 is 4wn to 20cm.
is the appropriate length.

Next, the reflectance of the mirror 4 will be described. From Figure 3,
It can be seen that sufficient deviation can be suppressed if the intensity of the reflected light is several percent or more of the intensity of the emitted light.

Therefore, the coupling efficiency between the LD and the tip fiber is 4o.
%, and assuming that the coupling efficiency in the reverse direction is 10 to 40%, if the reflectance of the reflecting mirror is 30% or less, the effect will be small because the reflected light intensity ratio to the LD will be 6% or less. On the other hand, when the reflectance is 9Q% or more, the intensity of light transmitted through the reflection mirror becomes small, resulting in a reduction in transmission power.

Next, the optical fiber for transmission will be described. The transmission optical fiber used has all end faces slanted to prevent light reflected from the end face from returning to the LD. Incidentally, one end of the pig-tilt fiber 3 is made vertical and is coated with a reflective coating to return reflected light to the LD to form an external cavity. The output light of this pig-til fiber 3 is guided to the core 104 of the multi-mode fiber 6 with an oblique end face, but since the core diameter of the pig-til fiber 3 is 10 μm or less and the core diameter of the multi-mode fiber is 60 μm, this coupling The loss in the area is very small. Furthermore, since one side of this coupling part is vertical and the other side is oblique, fluctuations in distortion of transmitted light due to multiple reflections, which is a problem in normal connector coupling of vertical end faces, do not occur.

If the number of pig till fibers 3 is several pieces or less, the LDl and the pig till fiber 3 are assembled on the same substrate,
It is very easy to use if it is made into one module and a diagonally polished connector is used for the transmission fiber. Furthermore, in the case of a long pigtil fiber, it is possible to configure the output end as a connector and connect it to a diagonal polished connector for transmission via an adapter.

FIG. 4 shows an example in which the present invention is applied to the transmission of frequency-multiplexed TV signals as transmission signals. The 1"V signal (multiple AH) received by the antenna 6 is level-matched and multiplexed for independent broadcasting signals by the ID head end (HE) 7, then amplified by the amplifier 8 and applied to the LDl of the LD module 9. Ru.

This module consists of the LD1, the above-mentioned pig-tilt fiber 3, a photodetector 12 for APC (automatic power control), and an optical adapter 102 for optical output. The output of the photodetector 12 enters the APC circuit 13, which controls the LD bias current IB and makes the LD output constant.The LD module 9 is warmly stabilized by the ATC (automatic temperature control) circuit 11, and the LD characteristics are stabilized. We aim to make this possible. The optical output of the module 9 is coupled to a transmission optical fiber 16 by an adapter 1o and an optical connector 14 whose end face is slanted. The output end of the transmission optical fiber is connected to a light receiving module 17 by a connector 16 whose end face is polished diagonally, and the output is amplified by an amplifier 18.
It is sent to the V receiver.

Although this embodiment is for the transmission of TV signals, it is obvious that it can also be applied to other signal transmissions.

In addition to the effects of the invention, in the LD optical transmission device of the present invention, since the coupling part with the external resonator is also used as the coupling part with the fiber, it is possible to manufacture an LD module with an external cavity with a simple configuration. , since the amount of LD wavelength shift due to LD direct current modulation is suppressed, it is possible to suppress signal distortion during multimode fiber transmission.Furthermore, since the LD oscillates in a single mode, LD noise can be suppressed. It becomes possible to transmit very high quality signals with less generation and suppressed modal noise.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of an LD optical transmission device according to an embodiment of the present invention, - Fig. 2 is a configuration diagram of an external resonator in the same LD optical transmission device, and Fig. 3 is a diagram of the LD
Figure 4 is a characteristic diagram showing the degree of suppression of the oscillation wavelength shift of the LD.
FIG. 1 is a configuration diagram of a TV signal transmission device to which an optical transmission device is applied. 1... LD, 2... Stem, 3... Groaning fiber, 4... Mountain/mirror coat end, 5...
Multimode fiber, 1o1...Active layer, 1
o2...core, 1o3...tapered ball, 1o4...core. . Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2! Figure 3-6:

Claims (3)

[Claims] (1) An LD whose brightness is directly modulated and the LD output light is guided to a short single-mode fiber whose one end is reflectively coated with a vertical end face, and the optical output from the reflective coated end face is converted into a transmission multi-mode fiber whose end face is oblique. An LD optical transmission device characterized in that the other end face of the multimode fiber is also made oblique. (2) The LD optical transmission device according to claim 1, wherein a single mode fiber having a length of 4 mm or more and 20 cm or less is used. (3) The end face intensity reflectance of the single mode fiber is 30
% to 90%. LD optical transmission device according to claim 1.