JPH03144614A - Optical transmitter - Google Patents

Abstract

PURPOSE:To eliminate waveform deterioration, crosstalk, etc., by composing the optical trans mitter of polarization modulators which modulate the polarization state of signal light outputted from a transmitting light source according to a modulating signal, an optical ampli fier, and a polarizer which extracts only a specific polarized light component from the signal light output from the optical amplifier. CONSTITUTION:The signal light 2 output from the signal light source 1 is collimated by a lens 3 into collimated light, which passes through a polarization modulator 4 and is then made incident on a polarization modulator 5. In the polarization modulator 5, the modulating signal 6 with a bit rate of 100Mb/s is applied to KDP crystal to generate a phase difference between polarized light components in the direction of an ordinary and an extraordinary optical axis and the polarization state of the signal light 2 at the time of the output of the polarization modulator 5 changes. The signal 2 which is output by the polarization modulator 5 is converged by a lens 7 and inputted to an optical amplifier 8, the signal light 2 output by the optical amplifier 8 has its modulation in the polarization state converted into intensity modulation by the polarizer 9, and the signal light 2 passed through the polarizer 9 is con verged by a lens 10 and made incident on an optical fiber 11. Consequently, even when the intensity of the signal light is large, neither waveform distortion nor crosstalk is caused.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical transmitter in the fields of optical communication, optical switching, optical information processing, etc.

(Prior art) In order to extend the distance that can be transmitted in optical communication, or to increase the number of signal lights distributed to multiple optical receivers, it is necessary to increase the transmission output of the optical transmitter. It is valid. To this end, it is effective to use an optical amplifier as a booster amplifier to amplify the signal light output from the transmission light source. In other words, Hagimoto et al. modulated the injection current of a signal light source (1,552μll0FB-LD) in an optical transmitter at a bit rate of 1.8 Gb/s, and transferred the signal light output from this signal light source to an Erbium Beam 1 optical fiber. By amplifying with an amplifier, 11.1 dol
The transmitter output of
1212 +o non-repeat 1F transmissiten exvariment atte 1.80b/S using eld erbium F%-IF fiber
Direct Detection Repeater System (A
212Kn non-repeated tra
nsllission exper+-nent
at 1.8Gb/s using LD
pulped Er -doped fiber
aIplifiers in an IN/di
rect-detection repeater 5
ystel) J, OFC89
(OFC89), Post Deadline Heber
(post deadline paper)
, PO 15, 1989).

Particularly in recent years, external modulation methods have attracted attention in order to avoid deterioration in receiving sensitivity due to dispersion of optical fibers. However, when an external modulation method is used, an insertion loss occurs in the external modulator, so the use of an optical amplifier is also effective from the point of view of compensating for this insertion loss.

(Problems to be Solved by the Invention) When using an optical amplifier as a booster amplifier in an optical transmitter, it is necessary to amplify extremely strong signal light. However, in conventional optical transmitters using optical amplifiers, the signal light is intensity-modulated before being input to the optical amplifier.
Waveform deterioration and crosstalk occur within the optical amplifier due to this intensity modulation of the signal light.

For example, in a semiconductor optical amplifier, when the signal light intensity exceeds the saturation level, the carrier density changes significantly depending on whether the signal light is ON or OFF. Therefore, the amplification factor after the modulation signal is continuously 1 (that is, the signal light is continuously ON) is
The amplification factor after the modulation signal continues for 3tL and O (that is, the signal light continues to be 0FF) is different, and a so-called pattern effect occurs.

Furthermore, even within a single pulse, the gain differs between the rising part of the pulse and after the pulse has risen, resulting in distortion of the pulse waveform. are very different. As a result, the refractive index of the semiconductor optical amplifier changes, and the wavelength of the signal light changes within one pulse.

This is a phenomenon similar to the chirping effect that occurs when a semiconductor laser is modulated, and similarly to the chirping effect, it causes sensitivity deterioration in an actual optical fiber transmission system due to the wavelength dispersion of the optical fiber.

Furthermore, in wavelength division multiplexing (WDM) optical communications, when a plurality of signal lights are amplified by one optical amplifier, crosstalk due to gain saturation becomes a problem. This is because when the sum of the intensities of multiple signal lights exceeds the saturation level of the semiconductor optical amplifier, the gain of the semiconductor optical amplifier is modulated by the signal light, which modulates other signal lights, resulting in crosstalk. It is something.

As described above, when an optical amplifier is used as a booster amplifier in an optical transmitter, since the signal light intensity is strong, waveform deterioration and crosstalk are likely to occur within the optical amplifier. An object of the present invention is to provide an optical transmitter using an optical amplifier that does not cause such problems.

(Means for Solving the Problems) In order to solve the above problems, one optical transmitter provided by the present invention includes a transmission light source and a polarization state of signal light outputted from the transmission light source according to a modulation signal. a polarization modulator that modulates the polarization modulator, an optical amplifier that amplifies the signal light output from the polarization modulator, and a polarizer that extracts only a predetermined polarization component from the signal light output from the optical amplifier. It is characterized by

Further, in order to solve the above problems, another optical transmitter provided by the present invention includes a transmitting light source that changes the frequency of output light according to a modulated signal, and a transmitting light source that amplifies the signal light output from the transmitting light source. It is characterized by comprising a polarization-maintaining fiber optical amplifier that converts frequency modulation into polarization modulation, and a polarizer that extracts only a predetermined polarization component of the signal light output from the polarization-maintaining fiber optical amplifier.

(Operation) In the present invention, the polarization state of signal light is modulated according to a modulation signal, and after optical amplification, surface light modulation of signal light is converted into intensity modulation. Therefore, the intensity of the signal light input to the optical amplifier is always constant. Therefore, even when the signal light intensity is strong, waveform distortion, crosstalk, etc. caused by intensity modulation of the signal light do not occur.

(Embodiment) FIG. 1 shows the constituent countries of a first embodiment of the present invention. In FIG. 1, the signal light 2 output from the signal light at is
The light is collimated by a lens 3, passes through a light leakage adjuster 4, and then enters a polarization modulator 5. In this embodiment, a 1.5 μm band DFB-LD is used as the signal light source 1.

Further, the polarization modulator 5 uses the electro-optic effect of a KDP crystal. The light leakage adjuster 4 is a combination of a 1/2 wavelength plate and a 174-wavelength plate, and by inputting light to this polarization adjuster 4, no matter what polarization state the input light is in, it can be changed to any polarization state. output light can be obtained.

The signal light 2 is converted into linear light having a polarization plane making an angle of 45 degrees clockwise with respect to the ordinary optical axis of the KDP crystal by the light leakage adjuster 4, and enters the light modulator 5. The polarization modulator 5 receives a modulation signal 6 with a bit rate of 100 Hb/s.
is applied to the KDP crystal, which causes a phase difference between the leakage components in the ordinary and extraordinary optical axis directions, and changes the polarization state of the signal light 2 when output from the polarization modulator 5. In this embodiment, the amplitude of the modulation signal 6 By setting the voltage to 20V, the modulation signal 6 becomes “1”.
”, a linear surface light was obtained which formed an angle of 45 degrees clockwise with respect to the ordinary optical axis, and when the modulation signal 6 was “O”, a linear surface light was obtained which formed an angle of 45 degrees counterclockwise with respect to the ordinary optical axis.

The signal light 2 output from the polarization modulator 5 is focused by a lens 7 and input to an optical amplifier 8.

As the optical amplifier 8, a 1.5 μ band semiconductor optical amplifier was used. The internal gain of this optical amplifier 8 was 25 dB, and the saturation input level at which the gain decreased by 3 dB was -13 dBri. This optical intensifier 8 is a traveling wave amplifier by applying a non-reflective coating to the end face, and the gain ripple is suppressed to 0.3 dB.

Furthermore, the gain difference between the TE and TM modes is suppressed to 0.8 dB. The modulation of the polarization state of the signal light 2 output from the optical amplifier 8 is converted into intensity modulation by the polarizer 9. In other words, by setting the polarization direction of the photon 9 so that a linear surface light forming an angle of 45 degrees with respect to the ordinary optical axis of the polarization modulator 5 passes, when the modulation signal 6 is "1", O
N, the signal light 2 which is OFF when the modulation signal 6 is "O" is output from the polarizer 9. The signal light 2 after passing through the polarizer 9 is condensed by the lens 10 and sent to the optical fiber which is the transmission path. 11.

In this embodiment, the output intensity of the signal light source 1 is 0 dBl.

Since the insertion loss in the polarization modulator 5 was 7 dB and the coupling loss at the input end of the optical amplifier 8 was 3 dB, the intensity of the signal light 2 input to the optical amplifier 8 was -10 dBn. This input intensity exceeded the saturation input level of the optical amplifier 8, and the gain of the optical amplifier 8 at this time was 21 dB. Furthermore, the total of the coupling loss at the output end of the optical amplifier 8 and the loss due to the polarizer 9 was 4 dB. Therefore, it was possible to input the signal light 2 with an intensity of 7 dBl into the optical fiber 11. Moreover, even though the signal light 2 exceeding the saturation input level was input to the optical amplifier 8, no waveform deterioration occurred.

FIG. 2 shows a second embodiment of the invention 41! It is a diagram. The second embodiment uses two channels of wavelength division multiplexing (WDM).
) It is an optical transmitter that performs communication. In Figure 2, signal light source 2
0.21 is 1.540μm, 1.546. c.
z m″C″C″ oscillation 1F8-10. Signal light source 2
The signal light 22.23 output from the lens 2
4.25 is used as collimator light, and optical isolator 26.
27. After passing through the light leakage regulators 28 and 29, the light enters the light leakage modulators 30 and 31.

The light leakage adjusters 28 and 29 and the polarization modulators 30 and 31 were the same as in the first embodiment. This polarization modulator 30
．． 31 has a bit rate of 150Hb/s, 2
00Hb/s modulation signal 32.33 is applied,
As a result, the signal lights 22 and 23 undergo polarization modulation similar to the first embodiment. @The signal light 22.23 output from the optical modulator 30.31 is multiplexed by the optical multiplexer 36,
Multiplexed light 37 is output. As the optical multiplexer 36, a polarization-maintaining optical fiber coupler in which two polarization-maintaining optical fibers were connected by melt-stretching was used. The input/output ports of this optical multiplexer 36 are polarization-maintaining optical fibers, and the light input from the two input ports is combined while maintaining its polarization state, and output from the output port. Polarization modulator 30.
The third beam and the optical multiplexer 36 are connected so that the ordinary optical axis of the input port of the optical multiplexer 36 forms an angle of 45 degrees clockwise with respect to the ordinary optical axis of the surface light modulator 30, 31. As a result, the signal light 22 component in the multiplexed light 37 at the output port of the optical multiplexer 36 is linearly polarized light parallel to the ordinary optical axis when the modulation signal 32 is 1, and perpendicular to the ordinary optical axis when the modulation signal 32 is O. Become. Similarly, the signal light 23 component in the multiplexed light 37 is also
When the modulation signal 33 is 1, it is parallel to the ordinary optical axis, and the modulation signal is 3
When 3 is 0, the light becomes linearly polarized perpendicular to the ordinary optical axis. The multiplexed light 37 output from the optical multiplexer 36 is focused by a lens 38 and input to the optical amplifier 8. The optical amplifier 8 is a 1.5μ band traveling wave type semiconductor optical amplifier of the same type as that used in the first embodiment. The multiplexed light 37 output from the optical amplifier 8 is input to a polarizer 9 whose polarization direction is set so that linearly polarized light parallel to the ordinary optical axis of the output port of the optical multiplexer 36 passes through. As a result, the signal light 22 component in the multiplexed light 37 is turned ON when the modulation signal 32 is 1, and turned OFF when the modulation signal 32 is 0. Similarly, when the modulated signal 33 of the signal light 23 components in the multiplexed light 37 is 1, it is ON, and when it is 0, it is OFF. The multiplexed light 37 that has passed through the polarizer 9 is condensed by a lens 10, passes through an optical isolator 39, and then enters an optical fiber if, which is a transmission path.

In this example, the output intensities of the signal light sources 20 and 21 are all 0 dB3, the insertion losses of the polarization modulators 30 and 31 are both 7 (1 B, the insertion loss of the optical multiplexer is 4 dB, and the optical amplifier 8
The coupling loss at the input end was 3 dB. Therefore, the intensity of the multiplexed light 37 input to the optical amplifier 8 was -116 Bl, and the gain of the optical amplifier 8 at this time was 21.5 dB. Furthermore, the coupling loss at the output end of the optical amplifier 8,
The total loss due to polarizer 9 was 4 dB. Therefore, it was possible to input the signal light 2 which was intensity-modulated with an intensity of 6.5 dBl to the optical fiber 11. Moreover, even though the multiplexed light 37 exceeding the saturation input level was input to the optical amplifier 8, no waveform deterioration or crosstalk between the signal lights 22 and 23 occurred.

FIG. 3 shows the constituent countries of a third embodiment of the present invention. In the third embodiment, by using an optical amplifier having birefringence, light leakage modulation and optical amplification are performed simultaneously in this optical amplifier. Wavelength 1.535μ as transmission light R1
A modulation signal 6 with a bit rate of 1001 (b/S) is applied to the bias current 40 of the FB-10 which oscillates at a speed of 100H2. However, since the amount of change in the bias current 40 due to the modulation signal 6 is very small, the intensity of the signal light 2 is almost constant.This frequency-modulated signal light 2 is 3 is used as collimator light, and optical isolator 42
, after passing through the polarization adjuster 4, the gicroic mirror 4
3 and outputted from the excitation light source 44.
and is input into the polarization-maintaining fiber optical amplifier 47. This planar optical preserving fiber optical amplifier 47 is a PANDA type polarization preserving optical fiber whose core is doped with Er.
The excitation light 45 with a wavelength of 1.485μ product produces a wavelength of 1.5
The signal light 2 with a product of 35μ is amplified. Such a fiber-type optical amplifier does not have a dependence of gain on light leakage. , its polarization state is adjusted by the polarization adjuster 4.In this way, the polarization state is adjusted by the polarization adjuster 4.In this way, the polarization state is adjusted by the polarization adjuster 4.
When light is incident, a phase difference occurs between the polarized light components in the ordinary and extraordinary optical axis directions due to the difference in refractive index. This phase difference changes depending on the wavelength of the incident light, so if the wavelength of the incident light is modulated, the polarization state of the output light changes according to this modulation. In this example, the wavelength of the signal light 2 is 1.535 μm.
, frequency shift due to modulation signal 6 ji110GH2, l?
The length of the N optical storage fiber optical amplifier 47 was 50 m.

At this time, the signal light 2 output from the polarization-maintaining fiber optical amplifier 47 forms an angle of 45 degrees clockwise with respect to the ordinary optical axis when the modulation signal 6 is 1, and with respect to the ordinary optical axis when the modulation signal 6 is 0. The light was linearly polarized at an angle of 45 degrees counterclockwise. The signal light 2 output from the polarization-preserving fiber optical amplifier 47 is input to the photon 9 whose light leakage direction is set to form an angle of 45 degrees clockwise with respect to the ordinary optical axis of the surface-preserving fiber optical amplifier 47. Ru. As a result, the signal light 2 becomes
When the modulation signal 6 is 1, it is ON, and when it is O, it is OFF. This intensity-modulated signal light 2 is focused by a lens 10,
The signal is input to the optical fiber 11 through the optical isolator 39 .

In this embodiment, the total insertion loss of the optical isolator 42, light leakage adjuster 4, and dichroic mirror 43 is 5 dB, and the input coupling loss of the polarization-maintaining fiber optical amplifier 47 is 2 dB.
It was dB. Further, the output intensities of the signal light source 1 and the excitation light source 44 were 0 dBn and 15 del, respectively. Therefore, the signal light 2 input to the polarization preserving fiber optical amplifier 47
, the intensities of the excitation light 45 are -7 dBi and 136 B, respectively.
It was l. At this time, the gain of the polarization maintaining fiber optical amplifier 47 for the signal light 2 was 15 dB. Furthermore, the sum of the coupling loss at the output end of the polarization preserving fiber optical amplifier 47, the loss due to the polarizer 9 and the optical isolator 39 is 3
(1B. Therefore, the optical fiber 11 has a strength of 5
It was possible to input signal light 2 of dBn.

FIG. 4 is a configuration diagram showing a fourth embodiment of the present invention. The fourth embodiment is a two-channel wavelength division multiplexing (WDM) transmitter in which polarization modulation and optical amplification are performed in an optical amplifier, as in the third embodiment. In FIG. 4, the signal light sources 20 and 21 are 1.535 am and 1.5 am, respectively.
This is a DFB-10 that oscillates at 36 μm"C". The bias current 50.51 has a bit rate of 150 Mb/s and 200 Hb through capacitors 52 and 53, respectively.
/s modulation signal 32.33 is added. The signal light 22.23 output from the signal light source 20.21 is frequency-modulated with a frequency shift amount of 10 GHz using this modulation signal 32.33.

This signal light 22.23 is made into collimated light by a lens 24.25, and then an optical isolator 26.27 and a polarization adjuster 2
After passing through 8.29, it is input to two input ports of the optical multiplexer 36. As the optical multiplexer 36, a polarization maintaining optical fiber coupler was used as in the second embodiment. At this time, the signal lights 22 and 23 are made into linearly polarized lights parallel to the ordinary optical axes of the two input ports of the optical multiplexer 36 by the two polarization adjusters 28. The multiplexed light 37 outputted from the optical multiplexer 36 is focused by the lens 38 and incident on the dichroic mirror 43, and the wavelength 1.485 outputted from the excitation light source 44 is
It is combined with the μm excitation light 45 and input to the leakage preserving fiber optical amplifier 47 . This polarization preserving fiber optical amplifier 4
7 is the same as in the third embodiment. However, by rotating the output port of the optical multiplexer 36 along the optical axis, the multiplexed light 37 enters the polarization-maintaining fiber optical amplifier 47 as linearly polarized light having an angle of 45 degrees with respect to the ordinary and extraordinary optical axes. There is. As a result, as in the third embodiment, the frequency modulation applied to the signal lights 22 and 23 is converted to polarization modulation. @The multiplexed light 37 output from the optical conserving fiber optical amplifier 47 is input to the polarizer 9 whose polarization direction is set to form an angle of 45 degrees clockwise with respect to the ordinary optical axis of the polarization conserving fiber optical amplifier 47. Ru. FIG. 5 shows the relationship between the intensity of the output light from the polarizer 9 and the wavelength of the input light to the polarization-maintaining fiber optical amplifier 47.

As shown in FIG. 5, the intensity of the output light from the polarizer 9 is 200% with respect to the wavelength of the light leakage preserving fiber optical amplifier 47.
It changes periodically at intervals of H2. Therefore, the wavelength of the signal light 22.23 is set to 1.535 μm and 1.536 μm, and the amount of frequency deviation due to the modulation signal 32.33 is 10
By setting GH2, the signal light 22 component in the multiplexed light 37 is ON when the modulation signal 32 is 1 and OFF when the modulation signal 32 is O, and the signal light 23 component in the multiplex light 37 is ON when the modulation signal 33 is 1. , it is OFF when it is 0. This polarizer 9
The multiplexed light 37 after passing through is condensed by a lens 10 and input into an optical fiber 11 which is a transmission path.

In this embodiment, optical isolators 26, 27, polarization adjusters 28, 29, optical multiplexer 36, dichroic mirror 43
The total insertion loss is 9dB for each channel,
The input coupling loss of the leakage preserving fiber optical amplifier 47 is 2 dB.
Met. Further, the output intensity of the signal light source 1 and the excitation light source 44 was 068 m and 5.15 dBl, respectively. Therefore, the multiplexed light 37 input to the polarization preserving fiber optical amplifier 47,
The intensities of the excitation light 45 were -8 dBl and 13 del, respectively. At this time, the gain of the leakage preserving fiber optical amplifier 1gH47 was 15.5 dB, and the total of the coupling loss at the output end of the polarization preserving fiber optical amplifier 47, the loss due to the polarizer 9 and the optical isolator 39 was 3 dB.

Therefore, the optical fiber 11 has a strength of 4. ! It was possible to input multiplexed light 37 of Mal.

(Effects of the Invention) As described in detail above, according to the present invention, the intensity of the signal light input to the optical amplifier/amplifier is always constant, so even when the signal light intensity is strong, waveform distortion, crosstalk, etc. does not occur,
Therefore, it was possible to provide an optical transmitter with a higher transmission output than the present invention.

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of the invention, FIG. 2 shows a second embodiment of the invention, FIG. 3 shows a third embodiment of the invention, and FIG.
The figures are top diagrams each showing a fourth practical example of the present invention. FIG. 5 is a diagram explaining the principle of the fourth embodiment of the present invention. 1.20.21. : Signal light source, 2.22.23; Signal light, 3, 7, 10, 24, 25, 34, 35, 46;
Lens, 4, 2829; Surface light adjuster, 5.30
．． 31; polarization modulator, 6.32.33: modulation signal, 8;
Optical amplifier, 9: Polarizer, 11; Optical fiber, 26
, 27, 39, 42; Optical isolator, 40:
Bias current, 41; Capacitor, 43;
Dichroic mirror, 44; Excitation light source, 45;
Pumping light, 47; polarization preserving fiber amplifier.

Claims (2)

[Claims] (1) a transmission light source, a polarization modulator that modulates the polarization state of the signal light output from the transmission light source according to a modulation signal, an optical amplifier that amplifies the signal light output from the polarization modulator; An optical transmitter comprising: a polarizer that extracts only a predetermined polarized component from a signal light output from an optical amplifier. (2) A transmitting light source that changes the frequency of output light according to a modulated signal, a polarization-maintaining fiber optical amplifier that amplifies the signal light output from the transmitting light source and converts frequency modulation into polarization modulation, and the polarization-preserving An optical transmitter comprising: a polarizer that extracts only a predetermined polarized component of signal light output from a fiber optical amplifier.