Wideband multichannel fiber lasers with output power...

Details Wideband multichannel fiber lasers with output power... Wideband multichannel fiber lasers with output power...

2000-01-14

2003-05-20

Leung, Quyen (Department: 2828)

Coherent light generators

Optical fiber laser

C372S020000, C372S032000, C372S070000, C385S024000

Reexamination Certificate

active

06567432

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a wideband multichannel fiber laser used in WDM (Wavelength Division Multiplexing) optical communication systems, more particularly, to a wideband multichannel fiber laser whose wavelength range is expansible to increase the number of lasing channel, and whose channel output powers are equalized, in order to provide a useful multichannel laser source having a wide bandwidth.
A multichannel fiber laser is an apparatus for simultaneously outputting multiple laser lights having different wavelengths with uniform wavelength interval from one fiber laser apparatus. Such a fiber laser can be used as a light source for signal transmission in a WDM optical communication system and for characterization and evaluation of WDM devises and apparatuses.
A laser is composed of a gain medium, a pumping light source and a resonator, alternatively called a laser cavity. Among various materials that can be used as a gain medium of a fiber laser, an erbium-doped fiber (EDF) is most widely used. The EDF has also been used to make an optical fiber amplifier in an optical fiber communication system, known as an erbium-doped fiber amplifier (EDFA). A semiconductor laser, that is commonly known as a laser diode, having a wavelength of 980 nm or 1480 nm is used as a pumping light source and a WDM coupler is used to launch the pumping light into the EDF. A conventional directional coupler is used as an output coupler that extracts the laser output from a laser cavity. All the components including the WDM coupler, the EDF and the directional coupler are fusion-spliced or connected to form a laser cavity.
In EDF, small signal gain is established in proportion to launched pump power. When the small signal gain becomes equal to cavity loss, laser oscillation takes place at the wavelength of peak gain in spectral domain. Once the laser oscillation takes place, the laser output power is determined so that the saturated gain of the EDF may be equal to cavity loss. Consequently, the small signal gain at the lasing threshold is always the same as the saturated gain above the lasing threshold.
An optical filter is required when the laser oscillation at a specific wavelength is desired out of the wide gain bandwidth of EDF. If the optical filter passes-a single wavelength only, it is called a single channel filter and the laser containing this filter oscillates in a single wavelength or a single channel. If the optical filter passes multiple wavelengths, it is called a multichannel filter and the laser containing this filter may oscillate in multiple wavelengths or multiple channels. However, since the gain spectrum of EDF is dominated by homogeneous line broadening at room temperature, an erbium-doped fiber laser (EDFL) tends to oscillate in a single channel even though the multichannel filter is used. Particularly, if an optical isolator is inserted in a ring cavity to enable unidirectional laser operation, spatial hole burning effect does not appear in the gain medium, so that the laser is highly favorable to single longitudinal mode oscillation. Therefore, suppressing the tendency toward single mode oscillation is a crucial technology to obtain multichannel laser oscillation from EDFL s.
Gain bandwidth of EDF reaches ~80 nm in 1550 nm wavelength range. If multichannel laser oscillation could be obtained with 100 GHz WDM channel spacing defined by ITU(International Telecommunications Union) from an EDFL, the maximum number of channel simultaneously available from one light source would be 100. Needless to say, all channels must satisfy the previously mentioned laser oscillation condition in order to realize such an efficient multichannel light source. For example, when the wavelength of the light selectively filtered by a multichannel filter is &lgr;i (i=1,2,3), the following equation 1 should be satisfied in logarithmic scale:
Gs
(&lgr;
i
)=
Lc
(&lgr;
i
)  (1),
where Gs and Lc represent saturated gain and cavity loss, respectively.
Generally, the cavity loss, Lc has little dependence on wavelength, which means Lc is almost a constant function of wavelength. The saturated gain, Gs, however, has an inherent spectral profile depending on both pump power and input signal power owing to saturation characteristics of a gain medium. Consequently, in an ordinary condition, the equation 1 cannot be satisfied at all wavelengths. In case of a homogeneously broadened gain medium such as EDF, if the equation 1 is first satisfied at the wavelength of peak gain and laser oscillation takes place at this wavelength, the gain profile remains unchanged even though the pump power increases, which results in single channel oscillation, and the equation 1 cannot be satisfied at the other wavelengths.
Therefore, to find out a way to make optical fiber lasers oscillate in multiple channels and to increase the number of useful channels is becoming important technology in the field of WDM optical communication.
There have been 4 major methods invented to obtain multichannel laser oscillation from optical fiber lasers.
The first method is, as shown in
FIG. 1
, to cool EDF down to extremely low temperature by using a cooling device such as liquid nitrogen in order to reduce the homogeneous linewidth down to 0.5 nm or less. When EDF has the homogeneous linewidth of less than 0.5 nm, the two lights whose wavelengths are separated by 0.5 nm or more can obtain independent gains from EDF, which results in simultaneous laser oscillation at the two wavelengths. A birefringence filter consisting of a polarization maintaining fiber(PMF)
5
, a polarizer
3
and a polarization controller (PC)
3
was used as a multichannel filter. The laser was made to oscillate unidirectionally using an optical isolator
6
. The laser output is obtained via a 10% coupler. This method has played an important role in analyzing the characteristics of EDFL s. However, this method has limited applicability since it uses a cooling device such as liquid nitrogen that is hard to maintain.
FIG. 2
shows a schematic of the second method of realizing multichannel fiber lasers. It uses a 1×N coupling device for branching a light into a plurality of optical paths, such as a multi-branch optical fiber coupler and an AWG(Arrayed Waveguide Grating) filter, and a N×1 coupling device for multiplexing the branched light. Each optical path between 1×N and N×1 coupler contains a piece of EDF and an optical tunable filter (TF) having different transmission wavelength so as to selectively pass and amplify the light of a specific wavelength defined by the filter. With this configuration, each light of the corresponding optical path is able to obtain independent optical gain, so that the laser as a whole may operate in multiple channels defined by TF s. This method allows the multichannel fiber laser to operate at room temperature and the wavelength stability and the output power of each channel can be controlled by the corresponding TF and variable optical attenuator (ATN), respectively, channel by channel. However, this method eventually makes the laser system complicated when the demand of channel number increases, because the addition of channel requires additional set of EDF, an optical tunable filter, and a variable optical attenuator.
The third method is, as shown in
FIG. 3
, to connect a number of single channel fiber lasers,
8
A-
8
B, with different wavelengths serially.
FIG. 3
shows a serially connected fiber laser only, but it is possible to connect fiber lasers parallel. An optical fiber DBR(Distributed Bragg Reflector) laser and an optical fiber DFB(Distributed Feed Back) laser may be used as the individual fiber laser. Both types of fiber lasers use FBG(Fiber Bragg Grating) technology. This method also allows the laser system to operate in room temperature, and has the advantage of simplicity in both concept and configuration. However, it requires an equipment for fabricating the FBG to make multichannel fiber lasers with specific channel spacing and channel nu

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