A relay station in an optical transmission system included in an optical communication network uses an optical amplifier that performs signal amplification directly on light in order to support higher velocities of (or wider bands for) optical signals instead of regenerative relay involving photoelectric conversion. Optical amplifiers presently generally used are optical amplifiers using rare-earth doped optical fiber as their amplifying medium. Among them, an optical amplifier (erbium doped fiber amplifier: EDFA) using erbium doped optical fiber (EDF) as its amplifying medium is mainly used.
FIGS. 15A and 15B illustrate a general EDFA. The EDFA illustrated in FIG. 15A includes an EDF 1a receiving input light O1 through one end, an excitation light source 2a that generates excitation light by using a laser diode (LD), and an optical coupler (such as a wavelength division multiplexing coupler or WDM coupler) 3a that provides excitation light from the excitation light source 2a from the one end to the EDF 1a. The EDFA applies front-pumping that transmits and amplifies excitation light that travels in the same direction as the direction of travel of the input light O1 to the EDF 1a. The EDFA illustrated in FIG. 15B includes an EDF 1b that receives input light through one end, an excitation light source 2b that generates excitation light by using a laser diode, for example, and an optical coupler 3b that provides excitation light from the excitation light source 1b to the EDF 1b through the other end of the EDF 1b. The EDFA applies back-pumping that transmits and amplifies excitation light that travels in the opposite direction of the direction of travel of the input light to the EDF 1b. In addition, bidirectional pumping EDFAs have excitation light sources both at the front and rear and excite an EDF bidirectionally.
As illustrated in FIG. 15A, during amplification by the EDF 1a, amplified spontaneous emission A1 occurs traveling in the opposite direction of the direction of travel of the input light O1. In order to suppress the reverse movement of the amplified spontaneous emission A1, an optical isolator may be provided. The same is also true for the EDF 1b and an EDFA of bidirectional pumping in FIG. 15B.
When such an EDFA is used to relay amplify an optical signal, polarization hole-burning (PHB) may occur in the EDF, a polarization dependent gain (PDG) may occur. Particularly, in a system having many relay stations using EDFAs on a transmission path, the effects of the polarization dependent gains relative to the EDFAs in the relay stations are accumulated. For example, when a signal band of C-band (1550 nm band: 1530 nm to 1565 nm) is relay amplified, the optical Signal-to-Noise ratio (OSNR) of the signal component on the short-wavelength side in the C-band may deteriorate unignorably. A signal band is a band in operation signal(s) that is continuously arranged wavelength(s) included in an input light. The input light includes at least one signal band. In recent optical networks, the transmission distances of both submarine and ground transmissions have increased. With the increases, the number of relays relative to optical amplifiers tends to increase. Solving the problems on the effects of the polarization dependent gains in the optical amplifiers now is important for the future.
Polarization hole-burning is a phenomenon having a gain that varies depending on the excitation light input to an EDF and the polarized state of signal light. When signal light with high intensity and degree of polarization (DOP) is input to an EDF, polarization hole-burning trims the gain of light in the parallel direction of polarization to the direction of polarization of the input signal light. The variation in gain in the EDF also affects amplified spontaneous emission (ASE) occurring in the EDF. The amplified spontaneous emission is unpolarized light and contains a parallel polarized component and a perpendicular polarized component to the direction of polarization of the signal light. Thus, the parallel polarized component to the signal light in the amplified spontaneous emission is affected by the gain variations due to the polarization hole-burning.
In other words, the polarization hole-burning occurring trims the gain of signal light and the gain of the polarized component parallel to the signal light in the amplified spontaneous emission. On the other hand, the polarization hole-burning occurring does not trim the gain of the polarized component perpendicular to signal light in the amplified spontaneous emission. Thus, regarding the amplified spontaneous emission occurring in an EDF, the difference between the gain relative to the polarized component parallel to the signal light with high degree of polarization and the gain relative to the polarized component perpendicular to the signal light is the polarization dependent gain. Therefore, comparing with the case without polarization hole-burning, the relative increase in proportion of the polarized component perpendicular to the signal light in the amplified spontaneous emission results in the reduction of the OSNR of the output light after the amplification. In other words, the signal light having a wavelength on the short-wavelength side of a C-band with high intensity and degree of polarization is affected by the polarization dependent gain due to polarization hole-burning. As a result, the OSNR after amplification decreases.
The polarization dependent gain due to polarization hole-burning depends on the degree of polarization of the light in an EDF. The higher the degree of polarizations, the more significantly the polarization hole-burning occurs. The term, “degree of polarization” refers to the ratio of optical power of a completely polarized component to the total optical power of light of the focused wavelength. The degree of polarization “0” indicates an unpolarized state, while the degree of polarization “1” indicates a completely-polarized state.
In a wavelength division multiplexing (WDM) optical transmission system that is a mainstream of the present optical communication, signal light beams having many wavelengths may be multiplexed for transmission, In this case, since the multiplexed signal light beams have various degrees of polarization, the degree of polarization of the entire WDM light is low. Thus, the influence of polarization hole-burning on polarization dependent gains is ignorably small. However, when a small number of signal light beams are to be multiplexed, for example, when signal light having one wavelength is only to be transmitted and particularly when the signal light positions on the short-wavelength side of the C-band, the influence as described above of polarization hole-burning on the polarization dependent gains become significant and unignorable.
The deterioration of the OSNR due to the polarization dependent gain when a small number of signal light beams are to be multiplexed will be described with reference to FIG. 15A. In the EDFA in FIG. 15A, it is assumed that input light O1 containing signal light S of one wave positioned on the short-wavelength side of the C-band and noise light N1 over the entire C-band is input to the EDFA. In this case, the input light O1 before amplification has an OSNR based on the power of a signal component in signal light wavelengths and the power of a noise component.
When the degree of polarization of the signal light S contained in the input light O1 is high, it is influenced by the polarization dependent gain due to the polarization hole-burning during the amplification in the EDF 1a. As a result, in the output light O2 after the amplification by the EDF 1a, the proportion of the noise component N2 increases particularly near the wavelength of the signal light S, and the OSNR relative to the signal light S decreases.
The OSNR may decrease for the same cause even with EDFAs of back-pumping in FIG. 15B and bidirectional pumping, not illustrated. In view of these situations, it may be important to suppress a polarization dependent gain in an optical amplifier applying rare-earth doped optical fiber.
The followings are reference documents.    [Document 1] Shoichi Sudo, “Erbium Doped Optical Fiber Amplifier”, The Optronics Co., Ltd., P. 59 to 61.    [Document 2] F. Bruyère, “Measurement of polarisation-dependent gain in EDFAs against input degree of polarisation and gain compression”, ELECTRONICS LETTERS 2 Mar. 1995 Vol. 31 No. 5, pp. 401-403.