Optical device and imaging system

Details Optical device and imaging system Optical device and imaging system

1999-11-03

2002-05-14

Mack, Ricky (Department: 2873)

Optical: systems and elements

Optical frequency converter

C372S020000

Reexamination Certificate

active

06388799

ABSTRACT:

FIELD OF INVENTION
The present invention relates to the field of optical devices such as radiation sources and detectors. More specifically, the present invention relates to optical devices used to efficiently generate a beam of radiation with a given frequency or within a band of frequencies. The present invention may also extend to an imaging system.
BACKGROUND OF INVENTION
Difficulties arise in producing electromagnetic radiation in several commercially important wavelength bands. For instance, in the terahertz (THz) frequency range (100 Ghz to 20 THz), near and mid-infra red (wavelength=1000-5000 nm) and blue/UV (wavelength=400-450 nm).
A possible method of producing such frequencies is to use non linear optical effects. The polarisation P in the material induced by the incident radiation can be expressed in terms of E the electric field exciting the material as the power series:
P=&khgr;
1
E+&khgr;
2
E
2
+&khgr;
3
E
3
. . .
Generally, the relationship P&agr;E is used and the higher order terms are assumed to be negligibly small. This approximation does not hold for large E. Non linear optics is concerned with these higher order terms.
If a material is irradiated with two different frequencies, the second order term allows the material to emit frequencies which are the sum of the input frequencies (known as Sum Frequency Generation), the difference between the input frequencies (Difference Frequency Generation) The second order susceptibility can also result in the generation of different optical frequencies when the material is irradiated by a single input frequency. For instance, second harmonic generation results from self sum generation. For optical parametric conversion, two frequencies are generated from the input frequency.
Also, the third order term &khgr;
3
E
3
can also be excited to produce third harmonic frequencies and other third order terms.
The problem which arises in such structures is that to obtain emitted radiation with the desired frequency efficiently, the phase of the polarisation induced by the incident radiation needs to be matched as closely as possible to the phase of the desired emitted radiation.
This phase matching problem also arises in certain types of detectors where changes in a reference beam with a first frequency due to a beam with a second frequency are used to measure the beam with the second frequency.
Some materials have a natural degree of phase matching due to birefringence properties and hence phase matching can be achieved over at least a certain length of the material. However, many materials with large optical non linearities nevertheless suffer from having no birefringence or other properties which allow some degree of phase matching and thus the full realisation of the material as a frequency converter.
SUMMARY OF THE INVENTION
The present invention addresses the above problems and in a first aspect, provides an optical device comprising an optical modulation region which comprises phase matching means for enhancing the phase matching between at least two different frequency signals propagating in the optical modulation region in response to illumination by at least one incident beam of radiation, the phase matching means having a spatial variation in its refractive index along a component of the incident radiation beam configured to maximise a distance in the modulation region over which the at least two different frequency signals stay in phase.
To clarify the above, a component of the incident radiation beam is a directional component of the beam.
If the present invention is used for frequency conversion, the optical modulation region preferably comprises frequency conversion means for emitting a beam with an emitted radiation frequency in response to illumination with the at least one incident beam of radiation, the phase matching means enhancing the phase matching between the polarisation generated by the or an incident beam and the emitted beam. The emitted beam having a different frequency to that of the incident beam.
There are many possible mechanisms for converting the frequency of the input radiation.
A new frequency can be generated from incident radiation with two or more frequencies, by so called sum or difference frequency generation.
The optical device is illuminated with radiation which either has two frequency components or the device is illuminated with two beams of radiation of different frequencies.
The higher fields obtainable by pulsed lasers allow the (progressively smaller) non linearities in the polarisation term to be accessed.
Preferably, a pulsed laser is used in order to generate the two incident frequencies. Also, the radiation pulse, produced by a pulsed laser inherently contains a number of different frequencies making it ideal for sum or difference frequency generation over a broad range.
However, with the enhanced phase matching achievable with the present invention, lower fields can be used and hence the two frequencies could be provided by two CW lasers operating at different frequencies. These provide a continuous beam which does not provide as high an electric field as a pulsed system although non-linear effects still may be accessed with CW lasers. CW lasers also have the advantage that they are presently more ubiquitous and commercially available than pulsed lasers.
The incident radiation generates a time-dependent polarisation via the second order non linearity of the material. A simplified view of the mechanism is to picture the electrons in the material as being on springs. The incident radiation causes the electrons to vibrate with frequencies corresponding to the incident frequencies, their sum and their difference.
Which frequency is emitted is dependent on the non-linear coefficients of the material at the fundamental frequency, and phase matching between the non-linear polarisation and generated/converted radiation at the sum/difference frequencies.
For sum frequency generation, the phase matching condition is given by
&Dgr;
k=k
(&ohgr;
1
)+
k
(&ohgr;
2
)−
k
(&ohgr;
3
)=0,
where &ohgr;
1
+&ohgr;
2
=&ohgr;
3
and k(&ohgr;) is the k-vector of the light in the material at frequency &ohgr;. For difference frequency generation the phase matching condition is given by
&Dgr;
k=k
(&ohgr;
1
)−
k
(&ohgr;
2
)−
k
(&ohgr;
3
)=0
where &ohgr;
1
−&ohgr;
2
=&ohgr;
3
.
For Nth harmonic generation the phase matching condition is given by
&Dgr;
k=Nk(&ohgr;
1
)−
k
(&ohgr;
2
)=0
where N &ohgr;
1
=&ohgr;
2
.
For optical parametric generation the phase matching condition is given by
&Dgr;
k=k
(&ohgr;
1
)−
k
(&ohgr;
2
)−
k
(&ohgr;
3
)=0
where &ohgr;
1
=&ohgr;
2
+&ohgr;
3
.
The coherence length, which can be thought of as the distance over which the generated field becomes out of phase with the driving field, is defined as
l
c
=2/&Dgr;
k
The refractive index, n, of the material is defined by n=kc/&ohgr;, where c is the speed of light in vacuum. Since the refractive index of many non-linear materials varies with the frequency &ohgr;, the coherence length is typically just a few microns. The present invention modifies the dispersion, i.e. the variation of n with &ohgr;, in order to satisfy the phase matching condition &Dgr;k=0.
Considering the case where THz radiation with a frequency &ohgr;
THz
is generated from two visible radiation frequencies &ohgr;
oph
using the technique of difference frequency generation. Phase matching for the generating THz radiation from the difference between the frequencies of the incident radiation is:
 &Dgr;
k=k
(&ohgr;
opt
+&ohgr;
THz
)−
k
(&ohgr;
opt
)−
k
(&ohgr;
THz
)=0.
The coherence length, l
c
, which is a measure of the distance over which the optically-induced non-linear polarisation and the generated THz electric fields remain in phase, is defined:
l
c
=&pgr;c
/(&ohgr;
THz
|&eegr;
vis
−&eegr;
THz
|)
where &ee

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