JP4911922B2 - Optical amplifier - Google Patents

Description

  The present invention relates to an optical amplifier applied to a gain-clamped PON repeater or the like that increases the transmission distance of a PON (Passive Optical Network) system.

  The PON system is a cost-effective system that shares an optical fiber transmission line and a station apparatus among a plurality of users. ITU-T Recommendation G. An optical access system based on the 983 series and equipped with a B-PON (Broadband PON) interface has been introduced. Then, a GE-PON (Gigabit Ethernet PON) system having a gigabit transmission capability based on IEEE802.3ah has started to be introduced. Furthermore, ITU-T Recommendation G. A G-PON (Gigabit-capable PON) system that is compliant with the 984 series and enables efficient transfer of full service has attracted attention mainly in North America. Thus, the PON system is spreading all over the world as a favorite of FTTx (FTTH and others).

  By the way, in order for a telecommunications carrier to provide services economically, it is necessary to efficiently accommodate as many customers as possible with one OLT (Optical Line Terminal) on the accommodation station side. Therefore, it is important to increase the distance between the OLT and the ONU (Optical Network Unit) in the home in an area where service requests are relatively small (depopulated area). On the other hand, in areas (urban areas) where service requests are relatively high, it is important to increase the number of splitter branches allowed. In other words, by extending the transmission distance, one OLT can provide services to more customers in remote locations, and by increasing the number of allowed branches, services can be provided to more customers in urban areas. Can do.

  3R function (Reshaping, Retiming, Regenerating) that expands the transmission distance of the PON system, regenerates a synchronous clock from an electrical signal once it is converted to an electrical signal, performs identification regeneration, and then converts it back to an optical signal PON repeater (3R repeater) (see Non-Patent Document 1, for example) and an optical amplifier. The 3R repeater has the advantage that an economical optical module can be obtained because it uses a standard-compliant optical interface. However, each 3R repeater has its own repeater for PON systems with different transmission bit rates and transmission protocols. Development is required. On the other hand, an optical amplifier has an advantage that an optical signal can be amplified regardless of a transmission bit rate or a protocol.

  By the way, in the PON system, since the transmission (downstream) signal from the OLT to the ONU is continuous light, optical amplification using existing technology is possible. On the other hand, the transmission (upstream) signal from the ONU to the OLT is a burst light, and since the distance between the OLT and each ONU is different, the OLT receives a burst optical signal having a different light intensity from each ONU. Even in a PON repeater using an optical amplifier, it is necessary to amplify these burst optical signals.

  However, when the burst optical signal is amplified by the optical amplifier, an optical surge is caused by the instantaneous gain characteristic. Optical surges not only prevent normal signal reception at the OLT, but can also cause optical receiver failure. Therefore, the optical amplifier needs some gain control in order to suppress the optical surge generated when the burst optical signal is amplified.

  There are mainly two methods for gain control of an optical amplifier. One is an automatic gain control (AGC) method that monitors the input / output light intensity and realizes a constant output operation by feedback / forward. The other is a gain clamp (GC) method in which gain control light is simultaneously amplified with signal light to suppress gain fluctuation due to burst behavior of input signal light. In particular, the AGC method is applied to many optical repeaters for long-distance transmission systems.

  However, the response speed of the AGC method depends not only on the speed of the control circuit but also on the instantaneous gain response characteristic of the optical amplifier. Therefore, the AGC method is too slow in response to a signal whose light intensity changes drastically like a burst signal. On the other hand, the GC method does not depend on the gain response characteristic or the speed of the control circuit. As a conventional 1.3 μm wavelength band optical amplifier that does not support burst, one made of praseodymium-doped fiber is known (for example, see Non-Patent Document 2).

  FIG. 7 shows a configuration of a gain clamp type PON repeater as a conventional GC control optical fiber optical amplifier. This PON repeater includes an optical isolator 31 on the input port side, an optical isolator 32 on the output port side, a GC control light source 33, a WDM (Wavelength Division Multiplexer) for multiplexing the GC control light λ2 and the signal light λ1. 34), a gain block 35 made of rare earth doped fiber, a pumping light source 36 for pumping the gain block 35, a WDM 37 for combining the signal light λ1 and the GC control light λ2 with the pumping light, and the signal light λ1 from the pumping light λ2. A WDM 38 for separation and an optical bandpass filter (BPF) 39 for separating the signal light λ1 from the spontaneous emission light noise and the GC control light λ2.

  In the PON repeater having such a configuration, in order to perform gain clamping, the wavelength of the GC control light λ2 is set to a wavelength within the gain band of the gain block 35. Then, by inputting the GC control light λ2 to the gain block 35 with a light intensity sufficiently higher than that of the signal light λ1, it is possible to realize optical amplification with a constant gain regardless of the change of the signal light intensity.

Kenichi Suzuki, Tetsuji Furukawa, Koichi Saito, Hiroshi Ueda, “Repeater Configuration Method for Realizing Expansion of Transmission Distance in B-PON”, Science B, J86-B Vol. 10, No. 2053-2064, 2003 October. Makoto Yamada, Terutoshi Kanamori, Yasutake Ohishi, Makoto Shimizu, Yukio Terunuma, Seiichiro Sato, and Shoichi Sudo, "Pr3 + -Doped Fluoride Fiber Amplifier Module Pumped by a Fiber Coupled Master Oscillator / Power Amplifier Laser Diode" IEEE Photon.Techol.Lett. VOL.9, NO.3, pp.321-323, March 1997.

  However, if the intensity of the GC control light λ2 is 10 dB to 20 dB higher than the signal light λ1, there is a drawback that the requirements for the BPF 39 for separating the signal light λ1 and the GC control light λ2 become severe. For example, in the BPF 39 having a GC control light suppression ratio of 20 dB, the optical signal λ1 and the GC control light λ2 have the same light intensity on the output side of the BPF.

  The present invention has been made in view of the above points, and an object of the present invention is to provide an optical amplifier capable of relaxing the requirements for the BPF for separating signal light and GC control light.

To achieve the above object, the invention according to claim 1, in and the signal light and different wavelengths within the amplification band of the gain block configured in praseodymium doped fiber, and the signal for each burst from an ONU of the PON system to the OLT of the light, by inputting the gain clamped optical burst optical signal by Redirecting a kina strength with maximum intensity from the rear to the gain block, and aims to obtain a constant gain regardless of the intensity of the signal light In the optical amplifier applied to the transmission signal from the ONU to the OLT of the PON system,
The gain block has a multi-stage configuration, and the signal light is transmitted from 1.29 μm to 1.31 μm to the final output stage, and spontaneous emission light of 1.27 μm to 1.28 μm or 1.32 μm to 1.35 μm. An optical bandpass filter for blocking noise and reflected light of the gain clamp light incident from behind is inserted, and the gain clamp light is reversely transmitted to each gain block through the circulator. It is characterized by inputting in the direction.

  According to the optical amplifier of the present invention, since the GC control light propagates in the opposite direction to the signal light and is input to the gain block, the GC control light component propagating in the same direction as the signal light becomes a reflection component of the GC control light and has an intensity. Decreases. For this reason, the requirements for the optical filter for separating the spontaneous emission light noise and the GC control light from the signal light can be relaxed.

  Hereinafter, an embodiment of a gain clamp type PON repeater to which the optical amplifier of the present invention is applied will be described.

  FIG. 1 is a block diagram illustrating a configuration of a gain clamp type PON repeater according to the first embodiment. In the first embodiment, an optical isolator 11 on the input port side, a gain block 12 made of a rare earth doped fiber, a pumping light source 13 for pumping the gain block 12, and signal light λ1 are combined with pumping light λ3 from the pumping light source 13. The WDM 14 for separating the excitation light from the signal light λ 1, the GC control light source 16, the circulator 17 for inputting the GC control light λ 2 from the GC control light source 16 to the gain block 12, and the natural light from the signal light λ 1. It comprises an optical bandpass filter (BPF) 18 for separating the emitted light noise and the GC control light λ2. The GC control light λ2 from the GC control light source 16 is propagated in the opposite direction to the signal light λ2 and input to the gain block 12, that is, the configuration of forward pumping and backward gain clamping.

  As described above, when the GC control light λ2 is propagated to the gain block 12 in the opposite direction to the signal light λ1, the component of the GC control light λ2 propagating in the same direction as the signal light λ1 is the reflected component of the GC control light λ2. It becomes. For this reason, the requirement (GC control light suppression ratio) for the BPF 18 for separating and removing the GC control light λ2 can be relaxed.

  As a specific example of the gain clamp type PON repeater of the first embodiment, when the wavelength of the signal light is 1.3 μm, the gain block 12 is composed of a praseodymium-doped fiber capable of amplifying light in the wavelength of 1.3 μm band. The excitation light source 13 is composed of an LD light source having a wavelength of 0.98 μm, and the GC control light source 16 is composed of an LD light source that oscillates at a wavelength of 1.27 μm to 1.28 μm or 1.32 μm to 1.35 μm. . Further, the BPF 18 is constituted by an optical bandpass filter having a passband with a wavelength of 1.29 μm to 1.31 μm.

  FIG. 2 is a diagram showing the configuration of the gain clamp type PON repeater according to the second embodiment of the present invention. The difference from the configuration of FIG. 1 is that the pumping light source 13 of FIG. 1 is removed, and pumping light from the pumping light source 13A disposed behind the gain block 12 is input to the gain block 12 by the WDM 15 in the opposite direction to the signal light. Thus, the WDM 14 is configured to separate the excitation light, that is, the configuration of backward pumping and backward gain clamping. Also here, the same praseodymium-doped fiber as in the first embodiment, the LD light source of the same wavelength, and the optical bandpass filter can be applied to the gain block 12, the excitation light source 13A, the GC control light source 16, and the BPF 18.

  FIG. 3 is a diagram showing the configuration of the gain clamp type PON repeater according to the third embodiment of the present invention. The difference from FIG. 1 is that the pump light from the pump light source 13 is input from the WDM 14 to the gain block 12, and the pump light from the pump light source 13A is input from the WDM 15 to the gain block 12 in the opposite direction to the signal light. And it is set as the structure of a rear gain clamp. Also here, the same praseodymium-doped fiber as in the first embodiment, the LD light source having the same wavelength, and the optical bandpass filter can be applied to the gain block 12, the excitation light sources 13, 13A, the GC control light source 16, and the BPF 18.

  FIG. 4 is a block diagram showing the configuration of the gain clamp type PON repeater according to the fourth embodiment of the present invention. This is a configuration in which the configuration of the first embodiment shown in FIG. 1 is arranged at the front stage, and the configuration of the second embodiment shown in FIG. 12A is a gain block, 14A and 15A are WDM, 16A is a GC control light source, and 17A is a circulator. Here again, the gain blocks 12 and 12A, the excitation light sources 13 and 13A, the GC control light sources 16 and 16A, and the BPF 18 are applied with the same praseodymium-doped fiber, the LD light source with the same wavelength, and the optical bandpass filter as in the first embodiment. it can.

  FIG. 5 shows gain characteristics of the gain clamp type PON repeater having the two-stage configuration shown in FIG. Here, the gain of signal light having a wavelength of 1.3 μm is 11.67 dBm, whereas the gain of GC control light having a wavelength of 1.32 μm is greatly reduced to −25 dBm. FIG. 6 shows the gain characteristics of a gain-clamped PON repeater having the same wavelength condition as the conventional configuration of FIG. Here, the gain of the signal light having a wavelength of 1.3 μm is 9.52 dBm, whereas the gain of the GC control light having a wavelength of 1.32 μm is substantially the same as 5 dBm. Is not enough.

  In the first to fourth embodiments, the case where the wavelength of the GC control light is set to be larger than the wavelength of the signal light has been described. However, the GC control light is within the signal light amplification band (amplification of the gain blocks 12, 12A). The wavelength may be smaller than the wavelength of the signal light as long as it has a wavelength different from that of the signal light and has a sufficiently large intensity with respect to the signal light.

It is a block diagram which shows the structure of the gain clamp type PON repeater of Example 1 of this invention. It is a block diagram which shows the structure of the gain clamp type PON repeater of Example 2 of this invention. It is a block diagram which shows the structure of the gain clamp type PON repeater of Example 3 of this invention. It is a block diagram which shows the structure of the gain clamp type PON repeater of Example 4 of this invention. It is a gain characteristic figure of the gain clamp type PON repeater of Example 4 of the present invention. It is a gain characteristic figure of the gain clamp type PON repeater of the conventional 2 step | paragraph structure. It is a block diagram which shows the structure of the conventional gain clamp type PON repeater.

Explanation of symbols

11: Optical isolator 12, 12A: Gain block 13, 13A: Excitation light source 14, 14A, 15, 15A: WDM
16, 16A: GC control light source 17, 17A: Circulator 18: Optical band pass filter 31, 32: Optical isolator 33: GC control light source 34, 37, 38: WDM
35: Gain block 36: Excitation light source 39: Optical bandpass filter

Claims (1)

In and signal light and a different wavelength within the amplification band of the gain block configured in praseodymium doped fiber, and among the ONU of the PON system of the optical signal for each burst to OLT, I-out Redirecting a by burst optical signal having the maximum intensity An optical amplifier applied to a transmission signal from the ONU to the OLT of the PON system for obtaining a constant gain regardless of the intensity of the signal light by inputting the gain clamp light of the intensity into the gain block from behind. In
The gain block has a multi-stage configuration, and the signal light is transmitted through the final output stage at 1.29 μm to 1.31 μm and 1.27 μm to 1.28 μm or 1.32 μm to 1.35.
An optical bandpass filter for blocking the spontaneous emission light noise of μm and the reflected light of the gain clamp light incident from behind is inserted, and the gain clamp light is transmitted to the gain block through the circulator. An optical amplifier characterized in that the input is made in the direction opposite to the propagation direction.

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