Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2001-11-30
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S071000, C372S101000, C372S106000, C372S108000
Reexamination Certificate
active
06711185
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to coupling out light e.g. from a laser cavity.
2. Discussion of the Background Art
Today, most DWDM component test systems for high dynamic range are based on a tunable laser source (TLS) that uses a low source spontaneous emission (SSE) optical output. A laser beam may have an improved signal to noise ratio (SNR), if it is coupled out just behind a wavelength selective device as disclosed e.g. in EP-A-921614. Such output shall be referred to the following as “Low-SSE output”.
SUMMARY OF THE INVENTION
It is an object of the invention to further improve optical laser systems where light is coupled out, preferably in cavity systems to deliver maximum output power at minimum SSE. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
The main problem when using a beam splitter as an output coupler in a cavity arrangement is that not only the desired beam (e.g. the Low-SSE output) from one direction will be coupled out but also a comparable amount of light propagating in the other direction is coupled out on the opposite side of the beam splitter. This ‘undesired’ output might be utilized or not, but will nevertheless weaken the laser's performance.
The present invention provides a tool allowing to direction sensitive coupling out light, so that light travelling in different directions can be coupled out with different coupling ratios.
According to the invention, a direction sensitive coupling out of light is provided by an out-coupling arrangement having a first polarization converter, a polarization dependent coupling device, and a second polarization converter.
In operation when a first light beam propagating in a first direction is coupled to the out-coupling arrangement, the first light beam having a first state of polarization is launched to the first polarization converter for converting the first state of polarization in a first way to a second state of polarization. The first light beam with the second state of polarization is then launched to the polarization dependent coupling device, such as a polarization dependent beam splitter. The polarization dependent coupling device is provided for coupling out a portion of its incoming light beam, whereby the portion of the coupled out light is defined by the state of polarization of the incoming light beam. Dependent on the second state of polarization, a portion of the first light beam will be coupled out by the polarization dependent coupling device as a first output beam. The remaining portion of the first light beam is launched to the second polarization converter for converting the second state of polarization in a second way to a third state of polarization.
In operation when a second light beam propagating in a second direction different to the first direction is coupled to the out-coupling arrangement, the second light beam having a fourth state of polarization is launched to the second polarization converter for converting the fourth state of polarization in the second way to a fifth state of polarization. The second light beam with the fifth state of polarization is then launched to the polarization dependent coupling device coupling out a portion of the second light beam as second output beam dependent on the fifth state of polarization. The remaining portion of the second light beam is launched to the first polarization converter converting the fifth state of polarization in the first way to a sixth state of polarization.
Thus, it becomes clear that the coupling ratios for the first and second output beams can be defined by adjusting and/or modifying at least one of the parameters: the polarization dependent coupling ratio of the polarization dependent coupling device, the first and fourth states of polarization, and the first and second polarization conversion ways provided by the first and second polarization converters.
In a preferred embodiment, wherein the first and second light beams are each linearly polarized, the first and second polarization converters are provided as polarization rotators. The first polarization rotator rotates the first state of polarization by a first rotation angle to the second state of polarization. Dependent on the second state of polarization, a portion of the first light beam will be coupled out by the polarization dependent coupling device as the first output beam, and the remaining portion of the first light beam is launched to the second polarization rotator for rotating the second state of polarization by a second rotation angle to the third state of polarization.
Accordingly, the second polarization rotator rotates the fourth state of polarization of the second light beam by the second rotation angle to the fifth state of polarization. The polarization dependent coupling device couples out a portion of the second light beam as the second output beam dependent on the fifth state of polarization. The remaining portion of the second light beam is launched to the first polarization rotator rotating the fifth state of polarization by the first rotation angle to the sixth state of polarization.
The coupling ratios for the first and second output beams can thus be defined by adjusting the polarization dependent coupling ratio of the polarization dependent beam splitter to the first and fourth states of polarization and correspondingly defining the first and second rotation angles.
In a preferred embodiment, the invention is employed in a cavity structure wherein light is travelling in two (opposite) directions between two end points. In case that the state of polarization is substantially maintained when returning from such end point, the first and the sixth state of polarization substantially match. Accordingly, the third and the fourth state of polarization will also match substantially. The first and second rotation angles are preferably both selected to be 45°, so that the state of polarization of the light in each direction is turned by 90° in total. Thus, the fourth state of polarization is 90° different from the first state of polarization, or in other words, the state of polarization of the incoming first light beam is 90° different from the state of polarization of the incoming second light beam (from the other side).
Due to the first and second rotation angles to be 45°, the polarization dependent beam splitter will receive light beams in the different directions with 90° difference in the state of polarization.
The polarization dependent beam splitter is preferably designed to provide two functions. The first function, as explained above, is the ‘standard’ function of a polarization dependent beam splitter, i.e. to divide the incoming light into a first portion having a first state of polarization and into a second portion having a second state of polarization with 90° difference to the first state of polarization. The second function is of a ‘normal’ beam splitter, i.e. to divide a light beam into two portion with a given ratio between the portions. This second function can be added to a ‘standard’ polarization dependent beam splitter (providing only the first function) e.g. by providing adequate material coatings on the beam splitting surfaces. Combining those two functions allows achieving a polarization dependent beam splitter transmitting the portion of the incoming light having the first state of polarization and coupling out only a part of the portion of the incoming light having the second state of polarization (with 90° difference to the first state of polarization), while the other part of the portion of the incoming light having the second state of polarization will be transmitted in the same way as the portion of the incoming light having the first state of polarization. A given coupling ratio determines the ratio between the two parts of the portion of the incoming light having the second state of polarization. The coupling ratio is preferably designed to be substantially smaller than 100%, e.g. in a range of 10-30%, so
Agilent Technologie,s Inc.
Ip Paul
Rodriguez Armando
LandOfFree
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