Frequency locker

Coherent light generators – Particular beam control device – Mode locking

Reexamination Certificate

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Details

C372S032000

Reexamination Certificate

active

06731659

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to coherent light generator systems, and more particularly to systems for controlling the frequency of light used in such systems. It is anticipated that a primary application of the present invention will be in telecommunications, but the present invention is also well suited to use in laboratory measurement and other fields.
BACKGROUND ART
The ability to measure and control light wavelength or frequency is highly useful in industry and basic research. The telecommunications industry provides one excellent example, and it will be used occasionally herein. [Since the speed of light is constant in a given medium, it should also be understood that the wavelength and the frequency of radiation have a fixed relationship. Thus, although it is possible to speak here of “wavelength locking” or “frequency locking,” for ease of reference “frequency locking” and variants thereof are used. The term “wavelength” is also used, but sparingly to denote where multiple wavelengths may be present and where multiple channel processing may be desirable.]
Numerous systems exist to measure frequency in some manner, and to control a light source to provide or maintain a specific frequency. These, however, suffer from a number of limitations. Co-pending U.S. application Ser. No. 09/798,721 by one of the present inventors for a “Light Frequency Locker,” incorporated herewith by reference, provides a discussion of single channel prior art and of prior art systems known to the present inventors that do not integrate the light source and the light frequency control mechanism, i.e., additional major novel aspects of the present invention. Accordingly, the following discussion provides background particularly germane to the invention of this patent application.
FIG. 1
(background art) is a perspective view of the major operational elements of a single channel frequency locker
10
. A laser light source (not shown) produces a laser beam
12
which is directed through a first beam splitter
14
to produce a control beam
16
. The laser light source may be quite removed from the locker
10
, as implied here, and various optical devices like fiber optic cable may be used to route the laser beam
12
into the first beam splitter
14
or to receive it as it exits and route it onward for use in some end application. Typically only a small portion of the laser beam
12
is “split” out in this manner and used as the control beam
16
.
The laser beam
12
has a single light wavelength of interest, although others may also be present so long as they do not significantly effect the operation of the locker
10
. In particular, in many applications the laser beam
12
is modulated to carry information. The purpose of the locker
10
is to lock the light frequency of the laser beam
12
to a desired frequency.
The control beam
16
is directed into a second beam splitter
18
to produce a reference beam
20
and a measurement beam
22
. The reference beam
20
is directed to a reference detector
24
, where it produces a reference signal
26
. The measurement beam
22
is directed to an interferometer
28
to produce an interference beam
30
. The interference beam
30
is then directed to an interference detector
32
, where it produces an interference signal
34
.
The beam splitters
14
,
18
, the detectors
24
,
32
, and the interferometer
28
may be conventional commercially available units. For example, the beam splitters
14
,
18
may be what are often termed “half-silvered mirrors,” although the reflective material may not be silver and the reflectivity to transmitting balance may not be half and half. The detectors
24
,
32
may be photodetectors, such as photodiodes.
Many types of devices are suitable for the interferometer
28
. An air-spaced Fabry-Perot etalon is shown in
FIG. 1
, but solid etalons or diffraction gratings are examples of other suitable devices. In particular, however, the interferometer
28
is chosen to produce a usable amount of interference for the desired frequency of the laser beam
12
.
A processing circuit
36
is further provided to receive both the reference signal
26
and the interference signal
34
and to produce a correction signal
38
. The reference signal
26
is representative of the “raw” light intensity in the control beam
16
at any given moment. In contrast, the interference signal
34
is representative of the light frequency in the control beam
16
, and thus also in the laser beam
12
. By combining these signals, typically using differential amplification techniques, the processing circuit
36
is able to normalize for intensity variation in the control beam
16
and to further determine if frequency variation, i.e. “drift,” has occurred. It then can produce the correction signal
38
accordingly.
The correction signal
38
is used as feedback to the laser light source to achieve frequency control as the laser beam
12
is being produced. In this manner any drift can be detected while it is still minor and can promptly be corrected for, thus “locking” the light frequency to the desired frequency. The above discussion of the single channel frequency locker
10
is brief and does not cover non-germane matters, like tuning to an initial light frequency, but rather is intended to serve as a basis for the following discussion.
The locker
10
shown in
FIG. 1
illustrates several points. It is a stand alone unit, physically separated from the laser light source it is used with. Historically this has been the case in this art. Laser modules, containing a laser and a mechanism to control its light frequency, have been produced as one physical unit while the frequency lockers that direct operation of the control mechanism have been separate physical units. The laser modules and locker units are then combined, typically by a designer for use in an end application.
This unfortunately has a number of disadvantages. The costs of this approach are unduly high. There is an added direct cost for using two different units, often from two different providers. Another consideration is the added indirect cost of designing combinations into end applications where there ultimately is only one problem to be solved: providing a frequency locked light source.
Of growing importance, also, is the ultimate form-factor of a frequency locked light source. Using two discrete parts tends to undesirably increase the surface area or volume required. Today minimizing the form-factor is important in many applications, particularly as many such applications use multiple frequency locked light sources together and the surface area and volume required for this becomes quite appreciable.
There are yet other disadvantages, such as minimizing counts of stocked spares, the economics of dealing with multiple vendors, and even operational interference between the units. For instance, if a laser module uses heating to adjust light frequency, waste heat from this can adversely effect the frequency locker unit. Similarly, if cooling is used, adjacent components may be cooled somewhat as well. It is not possible to catalog all of the possible disadvantages here, but those noted are major ones and they serve to make the point that using two devices to solve one problem may be unduly complex or expensive.
FIG. 2
(background art) is a schematic view of a single channel frequency locked light source
50
, essentially including the single channel frequency locker
10
of
FIG. 1. A
laser module
54
is now provided. It includes a laser chip
56
able to produce the laser beam
12
, with some collimating optics
58
also depicted.
The laser module
54
here includes a temperature unit
60
. Heating and cooling the laser chip
56
are common ways to adjust the frequency of the laser beam
12
. The temperature unit
60
here may do either, or even both across time as operating requirements change.
A control circuit
62
that is somewhat different than the processing circuit
36
of
FIG. 1
is shown, and the correction signal
38
is now one among

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