Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-08-31
2001-09-25
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S097000
Reexamination Certificate
active
06295308
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to lasers, and particularly to external cavity lasers for use as a transmitter in optical communications.
2. Technical Background
In dense wavelength division multiplexing (DWDM) system applications, the transmitter wavelength has to be locked to one of the International Telephone Union (ITU) standard wavelengths of an ITU grid to meet crosstalk specification and ensure reliable operation of the system over its lifetime (about 25 years). The lasing wavelength of a free running commercial distributed feedback (DFB) laser, determined by its built-in DFB grating and refractive index of the semiconductor waveguide, changes with temperature at a rate of 0.1 nm/° C.
FIG. 11
shows a wavelength-locked DFB laser demonstrated by Nortel Technology as described in the article by B. Villeneuve, H. B. Kim, M. Cyr and D. Gariepy, “A compact wavelength stabilization scheme for telecommunication transmitter,”
Digest of the LEOS Summer Topical Meetings, WDM Components Technology
, WD2, 19-20, Aug. 13-15, 1997, Montreal, Quebec, Canada. A slightly diverging beam of laser light
112
transmitted through a Fabry Perot filter or a single-cavity multilayer dielectric filter
114
is detected by two closely spaced photodetectors
116
acting as apertures. The photodetectors
116
are equally spaced from the centerline of a semiconductor source laser
118
. Each photodetector
116
captures a different but overlapping center portion of the total solid angle emitted by the divergent laser light source, as the filter
114
is aligned to control and monitor the transmission wavelengths. Two different spectral responses, offset in wavelengths according to their angular difference, are produced as shown in FIG.
12
. The difference or discrimination signal
222
is used by an operational amplifier
220
to control a heat sink temperature to lock the lasing wavelength to the ITU wavelength or center frequency &lgr;
0
.
FIG. 12
shows the ideal case where the wavelength offset between the two responses is roughly equal to their effective bandwidths such that the center frequency is centered at the ITU wavelength. However, to reduce cost, it is desirable to eliminate the extra external feedback parts of the operational amplifier
220
and photodiodes
116
needed for wavelength discrimination in this type of temperature control of a wavelenth-locked laser while maintaining or improving temperature reliability.
Filter-locked external cavity lasers as shown in
FIGS. 13 and 14
have recently been proposed and demonstrated. These lasers do not need the feedback control to monitor wavelengths since the center wavelength of its filters, made of a dielectric material, such as the fiber grating and the multilayer dielectric filter, has been demonstrated to be much less sensitive to temperature (<0.005 nm/° C.) than that of the semiconductor grating filter used in the DFB laser. A reflective Bragg grating written into the fiber establishes the precise lasing wavelength. One of the frequencies of the ITU grid is selected for the Bragg grating. The advantage of writing the frequency into the silica fiber is that the silica has a small coefficient of thermal expansion (about 5×10
−7
/° C.) and the resonant Bragg frequency changes can be made negligible by temperature compensation.
As seen in FIG.
13
and described in U.S. Pat. No. 5,844,926, a semiconductor laser diode chip
118
is provided with an anti-reflection (AR) coating
26
on one end facet
132
to which is optically coupled a length of optical fiber pigtail
134
in which there is a Bragg grating reflector
136
defining one end of a laser optical cavity whose other end is provided by the reflecting end facet
138
of the laser chip remote from the AR coated end facet. This Bragg grating reflector thus provide a means of locking the laser frequency.
Instead of using fiber, air can be substituted in the external cavity of FIG.
14
. Here and described in U.S. Pat. No. 5,434,874 and U.S. Pat. No. 5,870,417, a gain medium, such as the semiconductor (laser chip)
118
has both front
138
and back
132
facets, as in
FIG. 13
, where the back facet
132
has the anti-reflection coating
26
. Light
142
from the laser chip passes through the back facet
132
into an external air cavity. The cavity contains a tuning element
162
, such as a prism, mirror, filter, or grating, that reflects specific laser wavelengths back into the laser chip
118
. This round-trip light action
142
causes the laser to output selectable wavelengths
62
through the front facet
138
. Thus, the wavelength of light output
62
from the front
138
facet of the laser chip can be controlled by changing the angle of the grating, filter or other tuning element
162
. The cavity also contains a collimating lens
144
which directs light emitted from the rear facet
132
of the chip onto the grating, filter, or other tuning element
162
.
However, due to its long external cavity, the filter-locked laser can not be directly modulated at a high bit rate. The 3-dB modulation bandwidth decreases as the external passive cavity length increases as seen in FIG.
9
. For example, the direct modulation bandwidth of a semiconductor laser with a 300 &mgr;m cavity length is about 10 GHz. Therefore, it is difficult to directly modulate a filter-locked laser at a rate of 2.5 Gbit/sec and beyond since the external passive cavity length is in the order of 1 cm or longer. Moreover, the direct modulation response at the frequency (peak frequency) corresponding to the round trip time is significantly enhanced as shown in FIG.
15
. The peak frequency as a function of the external passive cavity length is shown in FIG.
10
. If the peak frequency is close to one of the harmonic frequencies of the signal, the signal will be distorted. An external modulator is thus needed for high speed modulation. Additionally, there is at least a cost saving reason to integrate an external cavity laser with an external modulator.
SUMMARY OF THE INVENTION
One aspect of the present invention is an optical transmitter which provides the benefits of both filter-locked and wavelenth-locked lasers is taught by modifying an external cavity for the integration of an external modulator. The external cavity provides a round-trip path for light travel. A substrate is connected to the external cavity where at least one gain element and an optical modulator are integral with the substrate. A partial reflector is also integral with the substrate and couples the at least one gain element with the optical modulator.
In another aspect, the present invention includes a saturable absorber integrated between the gain element and the partial reflector. In a further aspect of the invention, the substrate is a waveguide having a first anti-reflection (AR) coated facet and a second opposed AR coated facet on opposed ends of the waveguide.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.
REFERENCES:
patent: 4745606 (1988-05-01), Uehara et al.
patent: 4904045 (1990-02-01), Alferness et al.
patent: 4955006 (1990-09-01), Fukushima
patent: 5166940 (199
Agon Juliana
Arroyo Teresa M.
Corning Incorporated
Inzirillo Gioacchino
LandOfFree
Wavelength-locked external cavity lasers with an integrated... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Wavelength-locked external cavity lasers with an integrated..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Wavelength-locked external cavity lasers with an integrated... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2518995