Wolfgang Sohler, Hui Hu, Raimund Ricken, Viktor Quiring, Christoph Vannahme, Harald Herrmann, Daniel Büchter, Selim Reza, Werner Grundkötter, Sergey Orlov, Hubertus Suche, Rahman Nouroozi and Yoohong Min
Integrated Optical Devi ces in Lithium NiobateLithium niobate offers incredible
versatility as a substrate for
integrated optics. Researchers
have developed an array of new
optical devices based on this
material, including waveguide
structures, electro-optical
wavelength filters and polarization
controllers, lasers with remarkable
properties, nonlinear frequency
converters of exceptional
efficiency, ultrafast all-optical signal
processing devices and single
photon sources.
he goal of integrated optics is to combine many waveguide-based devices with different function-alities on a single substrate and connect them with optical channel guides. Such optical circuits can be designed for a number of applications
in optical communications, instrumentation and sensing—in much the same way that electrical circuits form the backbone of integrated electronics.
One of the material systems that is enabling integrated op-tics is the ferroelectric lithium niobate (LiNbO3, or LN), which has excellent electro-optical, acousto-optical and nonlinear properties. Moreover, it can be easily doped with laser-active ions and allows for simple fabrication of low-loss optical wave-guides.
Waveguide structuresOptical channel waveguides of very low propagation losses (down to 0.03 dB/cm at 1,550 nm) have been fabricated through the use of the reliable, well-known technique of Ti-indiffusion; such waveguides are basic structures of most of the devices presented in this article. Ti:LN channels are weakly guiding structures with relatively large waveguide cross-sections and, therefore, large mode distributions (typical: 3.4 µm 3 4.7 µm at 1,550 nm); this represents a limitation for the efficiency of electro-optical, nonlinear and laser devices. To improve the performance of these devices, investigators have developed new waveguide structures such as ridge guides, pho-tonic crystal guides and bent, periodically poled lithium niobate (PPLN) structures.
Due to a strongly confined mode distribution, ridge guides can enhance the efficiency of electro-optical and nonlinear ef-fects and lower the threshold of waveguide lasers. Therefore, we developed a chemical etching technique to fabricate high-qual-ity mono-mode ridge guides in Ti:LN with a width between 4.5 µm and 7 µm and a height up to 8 µm (IEEE Photon. Technol. Lett. 19, 417). Alternatively, the ridge can be fabricated first, followed by a Ti-indif-fusion into the ridge only. Smooth surfaces and side walls result in low propagation losses (TE: 0.08 dB/cm; TM: 0.25 dB/cm) at 1.55 µm wavelength.
For the same reason, the inductively coupled plasma (ICP)-reactive ion etching (RIE) tech-nique was optimized to fabricate 1.5-µm-wide photonic crystal waveguides in a proton-ex-changed surface layer of LN. The pore distance and diameter are 500 nm and 340 nm, respec-tively (J. Vac. Sci. Technol. A, 24, 1012).
The efficiency of nonlinear devices also strongly depends on the interaction length, which should be as long as possible. Therefore, we have developed Ti:PPLN structures with a 180° bend of a radius of curvature from 20- 36 mm and an overall length up to 200 mm. As the domains preferentially grow with boundaries parallel to the main crystallographic axes, their orientation had to be changed abruptly three times by 60°.
Wolfgang Sohler, Hui Hu, Raimund Ricken, Viktor Quiring, Christoph Vannahme, Harald Herrmann, Daniel Büchter, Selim Reza, Werner Grundkötter, Sergey Orlov, Hubertus Suche, Rahman Nouroozi and Yoohong Min
Integrated Optical Devi ces in Lithium Niobate Researchers have fabricated not only straight and bent
single waveguides to develop new devices, but also coupled waveguides in PPLN. Arrays of up to 101 coupled single-mode guides have been developed; these have led to peculiar proper-ties for linear light propagation and have also yielded impres-sive new results for nonlinear propagation. Investigators have
[ Waveguide structures based on lithium niobate ]
Scanning electron micrographs of a chemically etched ridge guide in Ti:LN (top left) and of a pe:LN photonic crystal waveguide (top right). Selectively etched surface of a bent Ti:PPLN waveguide section, where the domain orientation changes by 60°; domain periodicity is approximately 17 mm (bottom left). Optical micrograph of the end face of an array of 101 coupled Ti:PPLN waveguides (bottom right).
15 kV X 2,700 5 mm 42 39 SEI
30 kV X 15,000 1 mm 46 29 SEI
Test channel Waveguide array
LiNbO3 substrate
www.osa-opn.org26 | OPN January 2008
cal counterparts, have no moving parts. Unlike ring resonators for wavelength filtering, rotation rate sensors usually require a much larger diameter; this is due to the Sagnac effect, in which a nonreciprocal phase shift is induced proportional to the area enclosed by the ring.
We recently demonstrated the first ring resonator fabricated in LN for rotation rate sensing with a diameter of 60 mm (Proc. European Conference on Integrated Optics, Copenhagen, April 2007). It consists of a low-loss Ti:LN waveguide ring cavity and a straight waveguide tangential to the ring forming a directional coupler. Both ends of the straight guide were connected to optical fiber. The whole substrate was glued to a copper base plate, which was temperature-stabilized by a thermoelectric cooler/heater, and packaged in aluminum housing. In this way, light can be coupled into the resonator propagating clockwise or counter-clockwise. By slightly tuning the laser frequency (around 1,530 nm wavelength), we measured a resonator finesse of approximately 9 in both modes of operation, corresponding to a cavity Q of 2.2 3 106.
To measure the Sagnac-effect induced frequency shift of the cavity resonance toward higher (lower) frequencies for clockwise (counter-clockwise) operation, we operated the ring resonator in both directions simultaneously using a narrow line width laser. The frequency was modulated around a cavity resonance to allow a sophisticated signal processing technique with two lock-in amplifiers. In this way, the mid-frequency of the laser could be locked to a cavity resonance of the non-rotating ring.
Consequently, if rotated, the transmitted intensities for light propagating clockwise and counter-clockwise changed in op-posite directions. Therefore, the difference signal was propor-tional to the rotation rate; a corresponding result is shown in the figure (above, left) as a function of time, when the device was rotated with an oscillating rotation rate. In current work, the noise equivalent rotation rate is 0.1 rad/s—still far from the theoretical limit of 0.09 rad/h for an integration time of 1 s.
studied effects such as spatial soliton formation, parametric switching and wavelength conversion (Opto-Electronics Review 13, 113-21).
Ring resonator as optical gyroscopeRing resonators belong to the most basic structures of integrat-ed optics and have a wide range of potential applications. They can be used as a wavelength filter and (de-)multiplexer, but also as a Sagnac-interferometer to measure rotation rates. Such sen-sors are attractive devices, which, in contrast to their mechani-
We recently demonstrated the first ring resonator fabricated in LN for rotation rate sensing with a diameter of 60 mm.
[ Ring resonator fabricated in LN ]
Opened aluminum housing with pigtailed ring resonator on a copper base plate temperature-stabilized by a thermoelectric cooler/heater (top). Rotation rate measured with the ring resona-tor versus time (bottom).
Electrode structure on top of waveguide (left) and flat-top filter response of a programmable wavelength filter (center). Packaged and fiber-pigtailed PMD-compensator (right).
3
0
–3
t [s]0 1 2 3
Rot
atio
n ra
te [r
ad/s
]
[ Arbitrary polarization transformation ]
0
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T [d
B]
Quadrature electrode
1.2 mm
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Ground
3L/4 L/4
OPN January 2008 | 27
PMD-compensator and programmable wavelength filterPolarization mode dispersion (PMD) compensators and pro-grammable wavelength filters are new electro-optical devices that consist of a single straight waveguide with a large number of special electrodes on the waveguide surface operating as a series of electro-optic TM-TE converters. If driven by two voltages applied to the in-phase and quadrature electrodes, an arbitrary polarization transformation is possible.
We have developed devices that are up to 85 mm in length and that have 65 converter sections. By taking advantage of the birefringence of the material, these sections allow for the induc-tion of a differential group delay of up to 22 ps. This property can be used to compensate the PMD of a long fiber optical transmission link in a short LN device. Using a cascade of polarization transformers and differential group delay sections, one can gener-ate a PMD profile inverse to that of the transmis-sion line and compensate even higher-order effects (Electron. Lett. 35, 652). The higher the bit rate of an optical transmission system, the more stringent the requirement for PMD compensation becomes.
As we reported at the European Conference on Integrated Optics in April 2007, a similar device between crossed polarizers can be used as a pro-grammable wavelength filter. The spectral response is determined by the electro-optic coupling strength along the interaction length. The response can be tailored by applying an appropriate set of voltages to the converter electrodes. For instance, one can generate a flat-top response. Moreover, researchers have demonstrated strong sidelobe suppression and tunability of the filter as well.
Waveguide lasers In recent years, a whole family of Er-doped wave-guide lasers of excellent quality has been developed, emitting in the wavelength range of 1,530 to 1,603 nm. Research teams have reported free run-ning lasers of the Fabry-Pérot type, harmonically mode-locked lasers (5 ps/10 GHz), Q-switched la-sers (4 ns/1 kHz/1 kW), distributed Bragg reflector lasers, self-frequency doubling devices, and ring and acousto-optically tunable lasers (IEICE Transactions Electron E88-C, No. 5, 990-7).
Er:LiNbO3 is an excellent laser material for integrated optics. It can be easily fabricated in the surface layer of a LiNbO3 substrate by indiffusion of a thin vacuum-deposited Er layer. Afterwards, a single mode channel waveguide can be defined by the standard indiffusion technique of Ti-stripes. If
the laser is optically pumped by 1,480 nm radiation, the result is a wavelength-dependent gain of up to 2 dB/cm.
Additional doping by Fe allows one to define holographic waveguide gratings of excellent quality. Reflectivities of greater than 95 percent and a spectral halfwidth of the grating char-acteristic of less than 60 pm have enabled the development of narrow linewidth integrated optical distributed Bragg reflector (DBR) lasers, distributed feedback (DFB) lasers and coupled DBR-DFB lasers.
Recently, acousto-optically tunable lasers have been devel-oped with a tuning range of up to 47 nm around 1,550 nm; they can be operated as conventional lasers, but also as devices with a frequency shift after each roundtrip in the cavity.
We recently demonstrated the first ring resonator fabricated in LN for rotation rate sensing with a diameter of 60 mm.
[ Frequency-shifted feedback laser ]
(Top) Schematic of the frequency shifted feedback (FSF) laser with intra-cavity acoustooptical filter with Ti-indiffused optical waveguide structure in Er:LiNbO3. (Center) Photo of the FSF laser during operation. The lasing waveguide channel can be observed by the emission of upconverted green light. (Bottom) RF spectrum of the output of the FSF laser measured with a fast photodiode. The discrete frequency components result from the beating of the different lines of the moving frequency comb with a separation of 711 MHz (equal to the laser cavity FSR). RBW: resolution bandwidth, VBW: video bandwidth of the RF spectrum analyzer.
Acoustoopiticalpolarization converter/
frequency shifter Acousticalabsorber
Optical waveguide Dielectric
mirror
Laser emission
(TM)Acousto-
opitical filterPolarization splitter
94.2 mm
AR coating
Pump (TM)
Dielectric mirror
XY
Z
Er:LiNbO3
Ti:Er:LiNbO3
LiNbO3
TM
TE
–60
–70
–80
–90
RF frequency [GHz]0 5 10 15 20 25
Sp
ectr
al p
ower
[dB
m]
711 MHz RBW=1 MHzVBW=100 Hz
TE TM
www.osa-opn.org28 | OPN January 2008
Frequency shifted feedback (FSF) laser Such a frequency shifted feedback (FSF) laser has remarkable properties. It consists of an integrated acousto-optical wave-length filter incorporated in the Er-doped amplifier section and of dielectric end face mirrors defining the waveguide resonator. The wavelength filter is composed of two polarization splitters and an acousto-optical polarization converter with a tapered di-rectional coupler for surface acoustic waves (SAW) in between.
The transmitted wavelength is selected by the frequency of the SAW (about 170 MHz) inducing the acousto-optical polarization conversion. As this process is determined by phase matching, a narrow filter bandwidth of about 1 nm results, de-pending on the interaction length. Due to the interaction with a running SAW, a frequency shift of about 170 MHz is imposed on the optical wave each time it passes the acousto-optical filter. Therefore, after each round trip, the frequency of the intracavity laser field is shifted by two times the SAW frequency. The result is a relatively broad line width of the laser emission of about 180 pm (pump power dependent). The output power is very stable with time.
The laser’s highly resolved output spectrum consists of a comb of narrow lines of constant frequency spacing (cor-responding to the free spectral range of the laser cavity). This comb changes its frequencies as a whole with the enormous chirp rate of 2.4 3 1017 Hz/s. Therefore, the device is ideally suited for frequency domain ranging using a Michelson inter-ferometer. Researchers have demonstrated a spatial resolution of about 1.6 µm/kHz.
Optical parametric generators and oscillators Researchers have also made considerable progress in develop-ing integrated nonlinear devices in general and wavelength converters in particular. The common feature of such devices is the use of waveguides in PPLN substrates, which exploit the largest nonlinear coefficient d33 and quasi-phase matching. Due to the confinement of the optical fields within the cross-section of a low-loss narrow channel guide, there are high and nearly constant intensities along an interaction length of up to 20 cm.
As a result, a high efficiency of wavelength conversion can be expected—up to several orders of magnitude larger than for bulk optical devices. The following are two examples that represent the latest development of integrated optical paramet-ric generators (OPG) and optical parametric oscillators (OPO) for tunable mid-infrared (MIR) generation (2,700-3,500 nm) (“Mid-Infrared Coherent Sources and Applications,” NATO Science Series II, 2007). They will be used in spectroscopic ap-plications for trace gas analysis.
Optical parametric generators In their simplest versions, OPGs consist of a single Ti:PPLN channel guide pumped by a strong wave at about half the wave-
[ Optical parametric generation ]
(Top) Synchronously pumped OPO with external cavity mirrors. (Bottom) Measured power characteristics of the synchronously pumped OPO as average MIR output power versus average pump power at a specific point of operation (see inset).
[ Optical parametric oscillators ]
Total OPF output (peak) power versus coupled pump (peak) power in a pulsed mode of operation (6.4 ps pulses of differ-ent duty cycle, as given in the inset) compared with results of a theoretical analysis (dotted line).
Researchers have made considerable progress in developing integrated nonlinear devices in general and wavelength converters in particular.
lengths to be generated. Some pump photons decay spontane-ously into one signal and one idler photon (optical parametric fluorescence, or OPF) and are then amplified by parametric amplification; both processes are determined by energy and wave vector conservation.
As a result, signal and idler wavelengths depend on the pump wavelength and can therefore be tuned easily. For example, by changing the pump wavelength (lp) in the range of 1.54-1.58 µm in a device of 31.44 µm periodicity of the ferroelectric domain grating, one can tune the signal and idler wavelengths in the range of 2.8-3.4 µm. The spectral half-width of the emission is around 10 nm (dependent on the point of operation and the length of the device, which is 94 mm).
At low pump power, the conversion efficiency of an OPG is extremely small. However, it rises dramatically at power levels exceeding 1 W—theoretically approaching about 50 percent at 150 W corresponding to a strong pump depletion. Experi-mentally, the measured results (for pulsed operation) are lower mainly due to non-ideal pump pulses and some waveguide inhomogeneities.
Synchronously pumped OPOs As the parametric gain of an OPG (over-)compensates the inherent propagation losses of an optical mode already at rela-tively low pump power levels, optical feedback, as provided by a resonator, can give rise to optical parametric oscillation; the precondition is a low-loss resonator with mirrors of sufficiently high reflectivity. Depending on the mirror characteristics, one can design singly resonant (SR) or doubly resonant (DR; reso-nant for both signal and idler waves) OPOs; they mainly differ in their spectral fine tuning properties, threshold and conver-sion efficiency.
Both types of OPOs can also be operated in a special pulsed mode of operation with periodic pulses shorter than the round trip time within the cavity. If the pulse repetition frequency matches the inverse of the round trip time, short signal and idler pulses are gener-ated and resonantly amplified during each roundtrip, resulting in the emission of short MIR pulses of high peak power. This mode of operation is called synchronous pumping, suited for SR OPOs as well as DR OPOs.
We recently dem-onstrated synchronous pumping of a DR OPO pumped by 6-ps pulses (1.54-1.565 µm)
of up to 12 W peak power at a repetition rate of about 10 GHz, equivalent to an average power of up to 600 mW. As the repeti-tion time of the pump pulses was nearly fixed to 100 ps, an OPO of the precise length of 6.805 cm was developed, leading to 10 MIR pulses traveling inside the cavity simultaneously.
Optical parametric oscillation was observed above a threshold of 200 mW (average) coupled pump power (lp = 1,552.6 nm); at 600 mW, about 4 mW of MIR (average) power was gener-ated corresponding to about 80 mW peak power. The figure at the bottom of the facing page shows the power characteristics as signal and idler (average) power plotted versus the pump power. The results of the modelling calculations using the parameters, as given in the inset of the figure, result in a similar depen-dence, but with a lower threshold (62 mW) and higher output power (8 mW at 100 mW pump power).
All-optical signal processing devices
Recent research has yielded a variety of efficient integrated optical devices for ultrafast all-optical signal processing in the 1.5 µm wavelength range. By exploiting quasi-phase-matched second-order nonlinear interactions in Ti-indiffused or proton-exchanged waveguides in PPLN (Ti:PPLN, pe:PPLN), inves-tigators have demonstrated wavelength conversion, dispersion compensation, parametric amplification, wavelength-selective time division multiplexing, phase- and polarization-switching as well as spatial switching, mainly for applications in optical com-munications (OFC 2007, paper OME3, J. Lightwave Technol. 24, 2579).
Second harmonic generation (SHG), difference frequency generation (DFG), cascaded SHG and DFG (cSHG/DFG), sum frequency generation (SFG), and cascaded SFG and DFG (cSFG/DFG) have all been exploited to develop efficient wavelength-converters. For example, the top part of the figure on the top of p. 30 shows a scheme of a fiber-connected optical subsystem with a Ti:PPLN channel guide as core component
[ All-optical signal processing ]
Scheme of all-optical signal processing via nonlinear interactions in a Ti:PPLN channel guide. (See the example in the text of the optically tunable wavelength conversion by cSFG/DFG.)
Researchers have made considerable progress in developing integrated nonlinear devices in general and wavelength converters in particular.
17 mm
lp1
lp2
ls
lilsf
www.osa-opn.org30 | OPN January 2008
for polarization-independent wavelength conversion by cSHG/DFG. This device was the key component in a 21.4 Gbit/s (per channel) differential quadrature phase-shift keying (DQPSK) transmission experiment with 22 WDM channels over more than 10,000 km; it was used in the middle of the span for com-pensation of chromatic dispersion and nonlinear impairments (J. Lightwave Technol. 24, 54–64).
The bottom portion of the figure shows the output spectrum of the converter of about –9 dB conversion efficiency. A similar device was recently used to demonstrate wavelength conversion of QPSK signals of up to 320 Gbit/s without any power penalty (CLEO/QELS 2007, paper CThF1).
Even optically tunable wavelength-conversion could be demonstrated by exploiting cSFG/DFG using two independent pump waves. The first pump enables efficient SFG of a high bit rate signal, followed by DFG with the second pump. Tuning the wavelength of the second pump results in tuning the idler wavelength. A conversion efficiency from the (transmitted) sig-nal to the generated idler of –4.7 dB was achieved with pump power levels of roughly 275 mW.
Single photon devices
For quantum cryptography and quantum information tech-nology in general, integrated photonic devices have become increasingly important. They offer the best performance for the generation, manipulation and detection of single photons. For example, integrated sources for single photon pair generation are, in principle, OPGs; however, they are operated at very low pump power levels. They also consist of a Ti:PPLN waveguide, as shown in the figure on the facing page, which will be com-bined with a WDM-coupler and a polarization splitter on the same substrate to suppress the pump radiation at the output and to separate signal and idler photons according to their state of polarization.
The Ti:PPLN waveguide was designed for photon pair generation in the third telecommunication window of optical fibers—i.e., for wavelengths in the range of 1,530 to 1,570 nm. Again, the wavelength of the pump determines the wavelength of both signal and idler photons via energy conservation and phase matching. The figure also presents the measured signal and idler spectra for pumping at 756 nm with 10 mW power only. The power of the signal and idler photons is in the pW range. At such power, the detection of single photons becomes possible by avalanche photo diodes (APDs). In contrast to the OPGs for MIR generation, signal and idler photons have orthogonal polarizations. This is a consequence of the special periodicity chosen for the ferroelectric domain grating and of the chosen polarization of the pump.
Resonant devices are also under development. They will be able to emit photons of a bandwidth more than two orders of
(Top) Schematic set-up of a polarization-independent wave-length converter with Ti:PPLN channel guide. (Bottom) Output spectrum of the converter during multi-channel wavelength conversion by cSHG/DFG.
Output spectra showing all-optical tuning of the idler by chang-ing the wavelength of pump 2 exploiting cSFG/DFG.
Other groups have demonstrated frequency up-conversion of single photons to enable the use of more efficient Si-APDs as detectors.
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OPN January 2008 | 31
>> R. Noé et al. “Integrated optical LiNbO3 distributed polarisation mode dispersion compensator in 20Gbit/s transmission system,” Electron. Lett. 35(8), 652 (1999).
>> R. Iwanow et al. “Arrays of weakly coupled, periodically poled lithium niobate waveguides: beam propagation and discrete spatial quadratic solitons,” Opto-Electronics Review 13(2), 113-21 (2005).
>> W. Sohler et al. “Erbium-doped lithium niobate waveguide lasers,” IEICE Transactions Electron E88-C, No. 5, 990-997 (2005) (invited).
>> S.L. Jansen et al. “Optical phase conjugation for ultra long-haul phase-shift-keyed transmission,” J. Lightwave Technol. 24(1), 54-64 (2006).
>> C. Langrock et al. “All-Optical Signal Processing Using x(2) Nonlin-earities in Guided-Wave Devices,” J. Lightwave Technol. 24(7), 2579 (2006).
>> H. Hu et al. “Plasma etching of proton-exchanged lithium niobate,” J. Vac. Sci. Technol. A 24(4), 1012 (2006).
>> H. Herrmann et al. “Integrated Electro-Optic Filter with Programmable Spectral Response,” European Conference on Integrated Optics (ECIO’07), paper ThA1, Copenhagen, April 2007.
>> H. Hu et al. “Lithium niobate ridge waveguides fabricated by wet etch-ing,” IEEE Photon. Technol. Lett. 19(6), 417 (2007).
>> B. Huettl et al. “320 Gbit/s DQPSK All-Optical Wavelength Conversion using Periodically Poled LiNbO3,” Proc. Conf. on Lasers and Electro-Optics (CLEO ‘07), Baltimore/USA, May 2007, paper CThF1.
>> C. Vannahme et al. “Integrated optical Ti:LiNbO’ ring resonator for rota-tion rate sensing,” Proc. European Conference on Integrated Optics (ECIO 2007), Copenhagen, April 2007, paper WE1.
>> W. Sohler et al. “All-Optical Signal Processing Devices with (Periodically Poled) Lithium Niobate Waveguides,” in Optical Fiber Communication Conference (on CD-ROM), Optical Society of America, Washington, DC, 2007, paper OME3 (invited).
>> M.U. Staudt et al. “Fidelity of an optical memory based on stimulated photon echoes,” Phys. Rev. Lett. 98, 113601 (2007).
>> S. Orlov et al. “Mid infrared integrated optical parametric generators and oscillators with periodically poled Ti:LiNbO3 waveguides,” Springer Book “Mid-Infrared Coherent Sources and Applications,” M. Ebra-himzadeh and I.T. Sorokina, eds., NATO Science Series II: Mathemat-ics, Physics and Chemistry (to be published).
magnitude smaller than shown in our example. Other groups have demonstrated frequency up-conversion (SFG) of single photons to enable the use of more efficient Si-APDs as detec-tors. Furthermore, investigators are continuing their substantial work to develop memories for single photons stored for some time by the excitation of erbium ions in Ti:LN waveguides (Phys. Rev. Lett. 98, 113601).
Conclusions Researchers have come a long way in developing new waveguide structures and integrated optical devices in lithium niobate. It remains a great challenge to develop integrated optical circuits
[ Photon pair generation ]
Scheme of an integrated optical circuit for photon pair generation via parametric down-conversion in a Ti:PPLN channel waveguide with WDM-coupler and polarization splitter for pump suppression and polarization separation, respectively (left). Measured signal and idler spectra of generated photon pairs of orthogonal polarization (right). Inset: idler spectrum close to degeneracy.
Other groups have demonstrated frequency up-conversion of single photons to enable the use of more efficient Si-APDs as detectors.
0.8 nm
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of even higher functionality and complexity by combining many different devices on a single LN chip. t
The work presented in this article was conducted by the Applied Physics group of the University of Paderborn as part of a coordi-nated research program on “Integrated Optics in Lithium Niobate: New Devices, Circuits and Applications.” It was supported over many years by a special grant of the Deutsche Forschungsgemein-schaft, which is gratefully acknowledged.
[Wolfgang Sohler ([email protected]) and his colleagues are affiliated with the physics department at the University of Pader-born, Germany.]
