Optical: systems and elements – Holographic system or element – For synthetically generating a hologram
Reexamination Certificate
1998-08-20
2001-05-15
Chang, Audrey (Department: 2872)
Optical: systems and elements
Holographic system or element
For synthetically generating a hologram
C385S001000, C385S003000, C385S002000, C385S004000, C385S008000
Reexamination Certificate
active
06233070
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an optical system including components defining two or more optical paths for the transmission of light beams and an arrangement for varying the lengths of two selected optical paths and to a method of changing the lengths of the optical paths. If light beams are transmitted along the two selected optical paths and the length of each optical path is varied, the phase of each light beam will be changed.
Such an optical system may include two optical paths of different lengths joined together in parallel. If a light beam is transmitted to one (upstream) junction of the two paths it will divide into two component beams, one of which will be transmitted along one of the parallel paths and the other of which of which will be transmitted along the other of the parallel paths. The component beams will meet at the other (downstream) junction of the two optical paths. Since the optical paths followed by the two component beams are of different lengths there will be a phase difference between the two component beams when they meet at the downstream junction. The light beams will combine and interfere with each other in a manner dependent on the size of the phase difference.
BACKGROUND ART
It is known to vary the length of an optical path in a transparent medium and thereby to change the phase of a light beam being transmitted along the path by using a device known as a phase modulator. Such a device may be an integrated electrical device such as a PIN diode that comprises heavily doped regions (n-doped and p-doped respectively) adjacent to the optical path. By passing a current through the PIN diode, carriers are injected into the transparent medium forming the adjacent portion of the optical path so that the refractive index of that portion of the optical path is changed. This change of refractive index effectively results in a change in the length of the optical path. This change in the length of the optical path results in a change in the phase of a light beam being transmitted along the optical path.
Integrated passive or active silicon-on-insulator (SOI) waveguides forming optical paths have very broad applications. Active integrated optical elements coupled to the waveguides may be based on phase modulators, such as the PIN diode modulators described above. Many integrated optical devices such as interferometers, switches and amplitude attenuators can be made from this integrated phase modulator structure. When a phase modulator is coupled to one of the optical paths of an optical system as above having two optical paths the difference between the path lengths of the two optical paths can be varied. The nature of the interference between the two light beams will therefore be varied.
Most phase modulators, such as the PIN diode phase modulator described above, require the use of a current driver. Due to the nature of the current response in most phase modulators the change in the optical path length and the phase change is a non-linear function of the driving current. In addition, due to the scattering of the light by the carriers injected into the material of the optical path, a light beam transmitted through the portion of an optical path which is coupled to a phase modulator including a PIN diode also undergoes amplitude modulation.
Another type of phase modulator is a thermal phase modulator. In this type of modulator a voltage is applied to heating or cooling means change the temperature of the material so that the phase of a light beam being transmitted along the path is varied. As for the above-described PIN diode type of phase modulator, the change in phase is a non-linear function of the voltage applied.
Integrated optical systems that include phase modulators are, for example, used in sensor applications, where the sensor system depends on the phase modulator to demodulate or process the signal. Any non-linearity of the phase modulator is thus reproduced in the sensor system output. Therefore, the non-linearity of prior art phase modulators directly affects the accuracy of the sensor system. Complicated linearization circuits have therefore been necessary in prior art systems to compensate for this deficiency.
Because of the nature of some phase modulators, such as the PIN diode phase modulator described above, the driving current always flows through the modulator in only one direction. Therefore, these phase modulators can only be driven in one direction and a standard push pull method of operation in which current can be made to flow through the phase modulator in either of two directions is generally not possible to implement. Active optical systems such as interferometers and switches comprise two optical paths and a phase modulator is used in one of the paths to change the Optical Path Difference (OPD) between the two paths and thus change the optical properties of the system. The phase modulator can be coupled to either optical path for generating a similar effect on the OPD change of the system. Therefore, it is customary to use only one phase modulator in such interferometers or switches.
A second phase modulator can be added into the other optical path of the system. Nevertheless, due to the nature of a phase modulator, each phase modulator generates a similar effect on its associated optical path, i.e. it makes the path length shorter when the driving current is increased and makes the path length longer when the driving current is decreased. As a result, in known systems, the path changes resulting from the phase modulators substantially counteract each other. Therefore, in prior art optical systems where a second phase modulator has been used, it has generally been used as a backup phase modulator in series with the first phase modulator and in the same optical path.
The object of the present invention is to provide an optical system including components defining two or more optical paths for the transmission of light beams and having an improved arrangement for varying the lengths of the optical paths so as to change the phases of light beams transmitted along the optical paths.
DISCLOSURE OF THE INVENTION
According to one aspect of the invention an optical system comprises components defining at least two optical paths and an arrangement for changing the length of two selected optical paths including two phase modulators, one coupled to each of the selected optical paths, and a driving system for applying power to the phase modulators to drive them in the same direction and for changing the amounts of power applied to the phase modulators in opposite directions so as to change the length of each optical path in a different direction.
The changes in opposite directions of the amounts of power applied to the phase modulators are preferably equal.
The amounts of power applied to the phase modulators may be changed simultaneously or in succession.
According to another aspect of the invention there is provided a method of changing the lengths of two optical paths, each path comprising a phase modulator, the method comprising the steps of: applying power to the two phase modulators so as to drive the phase modulators in the same direction, and changing the amounts of power applied to the phase modulators in opposite directions so as to change the length of each optical path in a different direction.
If a light beam is transmitted along each selected optical path when the length of each optical path has been changed, each light beam will undergo a phase change. Normally the relationship between the change in amount of power applied to a phase modulator coupled to an optical path and the resultant change in the phase of a light beam transmitted along the optical path is non-linear. By combining together the phase changes in the light beams using the above arrangement of phase modulators and arrangement for applying power to the phase modulators the non-linearity is substantially cancelled. As a result, the relationship between the changes in the amounts of power applied to the phase modulators and the resulting chan
Lu Yicheng
Pechstedt Ralf-Dieter
Bookham Technology plc
Chang Audrey
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
LandOfFree
Optical system and method for changing the lengths of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical system and method for changing the lengths of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical system and method for changing the lengths of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2435140