Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2000-12-19
2004-08-31
Le, Hoanganh (Department: 2821)
Coherent light generators
Particular beam control device
Tuning
C372S029011, C359S199200
Reexamination Certificate
active
06785306
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength-tunable stabilized laser which can vary the wavelength of emitted laser light and which, in particular, can almost lock the wavelength of emitted laser light at a desired wavelength.
To construct multimedia networks in the future, ultralong distance, large-capacity optical communication apparatuses are now demanded. To realize such large capacity, wavelength division multiplexing (WDM) optical communication apparatuses are studied and developed because of their advantages such as capability of effectively utilizing the wide bandwidth and large capacity of the optical fiber.
In particular, WDM light source for WDM optical communication is required to output laser light at a plurality of wavelengths. Further, the wavelength spacing need to satisfy a standard, for example, the grid spacing defined for respective channels according to the ITU-T recommendation. WDM light source is studied and developed so as to satisfy such a requirement.
2. Description of the Related Art
Conventionally, in the case of WDM optical communication systems that perform communication by using a four-wave WDM optical signal, a WDM light source is provided with four semiconductor lasers that emit laser beams having different wavelengths or a single wavelength-tunable laser capable of varying the oscillation wavelength by changing the device temperature or the driving current. For example, the multiple quantumwell (MQW) DFB laser, the wavelength-tunable distributed Bragg reflection (DBR) laser, or the like is used as the wavelength-tunable laser.
In particular, the use of a wavelength-tunable laser provides an advantage that the number of semiconductor lasers used as regular-use light sources and reserve light sources are reduced in a WDM light source. For example, in a 32-wave WDM optical communication system, 32 semiconductor lasers are necessary as each of regular-use light sources and reserve light sources if one semiconductor laser is used for one wavelength. In contrast, where wavelength-tunable lasers each capable of emitting laser beams of four wavelengths are used, it is sufficient to use eight semiconductor lasers as each of regular-use light sources and reserve light sources (maximum case).
On the other hand, although in semiconductor lasers the diffraction grating pitch etc. are so designed that single-mode laser light having a predetermined wavelength is emitted in a steady state, oscillation at the predetermined wavelength does not necessary occur at the time of ignition. Even in a steady state, the oscillation wavelength is not always locked at the predetermined wavelength owing to fluctuations. In the case of wavelength-tunable lasers, which are also associated with the above phenomena, the oscillation wavelength needs to be locked at a targeted predetermined wavelength because they can oscillate at multiple wavelengths.
To lock the oscillation wavelength at a desired wavelength, a wavelength locking apparatus is used in WDM light sources.
Referring to
FIG. 21A
, laser light output from a wavelength-tunable laser
911
is input to a coupler (CPL)
912
that is provided in a multi-wavelength locking apparatus
905
and serves to branch input light into two parts. One branched laser light is output as output light of a WDM light source. The wavelength-tunable laser
911
is an MQW semiconductor laser and has, for example, a characteristic that the oscillation wavelength varies by 0.8 nm when the device temperature is changed by about 8° C.-10° C. Where a WDM optical signal contains four optical signals at wavelength spacing of 0.8 nm according to the ITU-T recommendation, the wavelength-tunable laser
911
can output laser beams of four wavelengths in a temperature range of about 30° C. and emits laser light having one of the four wavelengths by controlling the device temperature.
In the multi-wavelength locking apparatus
905
, the other laser light that has branched off at the coupler
912
is input to a coupler
913
for branching input light into two parts. One laser light that has branched off at the coupler
913
is input, via a Fabry-Pérot etalon filter (ET filter)
914
, to a first photodiode (PD)
915
for outputting a current in accordance with light intensity. The light intensity of the laser light is detected by the first photodiode
915
. The output value of the first PD
915
is represented by PDo1. The other laser light that has branched off at the coupler
913
is input to a second PD
916
, where its light intensity is detected. The output value of the second PD
916
is represented by PDo2.
In the ET filter
914
, wavelengths having extremum transmittance values are so set that the PDo1 value as normalized by the PDo2 value at an intended locking wavelength, that is, PDo1/PDo2, becomes a target value 0.5.
A control CPU
917
receives PDo1 and PDo2. The control CPU
917
generates a control signal to be used for locking the oscillation wavelength of the wavelength-tunable laser
911
based on these detection values and sends it to the wavelength-tunable laser
911
.
The WDM light source having the above configuration operates in the following manner and thereby locks the oscillation wavelength of the wavelength-tunable laser
911
at a ch
0
wavelength, for example.
After igniting the wavelength-tunable laser
911
, the control CPU
917
receives PDo1 and PDo2 and calculates PDo1/PDo2 (see FIGS.
21
A and
21
B). When PDo1/PDo2 is greater than the target value 0.5, the control CPU
911
controls the wavelength-tunable laser
911
so that the oscillation wavelength becomes longer by adjusting its device temperature. On the other hand, when PDo1/PDo2 at the time of the ignition is smaller than the target value 0.5, the control CPU
917
controls the wavelength-tunable laser
911
so that the oscillation wavelength becomes shorter. The wavelength-tunable laser
911
is controlled in this manner so that PDo1/PDo2 is always kept at 0.5 and its oscillation wavelength is thereby locked at the ch
0
wavelength.
Where the control CPU
917
controls the oscillation wavelength merely by performing the magnitude comparison between PDo1/PDo2 and the target value 0.5, the oscillation wavelength can be locked at the desired ch
0
wavelength when the wavelength-tunable laser
911
has ignited at a wavelength of any of points a-d in FIG.
21
B. However, when the wavelength-tunable laser
911
has ignited at a wavelength of point e or f, the oscillation wavelength is locked at a wavelength other than the ch
0
wavelength.
In view of the above, the control CPU
917
also controls the device temperature at the time of ignition in consideration of a range including the oscillation wavelength at the time of ignition of the wavelength-tunable laser
911
.
As described above, in the WDM light source, the oscillation wavelength can be locked at any of ch
1
, ch
2
, and ch
3
wavelengths in the same manner as at the ch
0
wavelength by taking the device temperature at the time of ignition into consideration.
For a wavelength at the time of laser ignition, a wavelength range where a wavelength locking apparatus can lock the laser oscillation wavelength at a desired wavelength is referred to as “locking range.”
The locking range width is determined by the FSR (free spectral range) of the ET filter
914
because PDo1/PDo2 has the same value as the wavelength shifts by the FSR as shown in FIG.
21
C. Therefore, to equalize the oscillation wavelengths to the wavelengths of optical signals of a WDM optical signal, the FSR of the ET filter
914
is set equal to the wavelength spacing of the WDM optical signal.
On the other hand, the transmittance-wavelength characteristic of the ET filter
914
depends on the temperature. As seen from
FIG. 22
, as the temperature increases, the transmittance-wavelength characteristic shifts to the longer wavelength side in parallel with the horizontal axis at a rate of about 0.095 nm/°C. In
FIG. 22
, the vertical axis represents the current value in &mgr;A (corresponding t
Kai Yutaka
Miyata Hideyuki
Le Hoang-anh
Staas & Halsey , LLP
Vu Jimmy T.
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