- Email: [email protected]
Detection of mid-IR radiation by sum frequency generation for free space optical communication
ARTICLE IN PRESS
Optics and Lasers in Engineering 43 (2005) 537–544
Detection of mid-IR radiation by sum frequency generation for free space optical communication Ketil Karstada,*, Andre! Stefanova,1, Mark Wegmullera, Hugo Zbindena, Nicolas Gisina, Thierry Aellenb, # Mattias Beckb, Je! rome Faistb a
Group of Applied Physics, University of Geneva, 1211 Geneva 4, Switzerland b # # Institute of Physics, University of Neuchatel, 2000 Neuchatel, Switzerland
Received 6 January 2004; received in revised form 5 May 2004; accepted 7 May 2004 Available online 4 August 2004
Abstract We present an experiment where mid-infrared radiation is detected indirectly via the secondorder non-linear process of sum frequency generation. The mid-infrared sources used for the experiment are quantum cascade lasers, and we use a pump wavelength that yields an upconverted wavelength within the detection window of Silicon avalanche photo diodes. Compared with direct detection using state-of-the-art mid-infrared semiconductor detectors, the detection scheme we propose in this paper has the advantages of greater bandwidth and lower noise equivalent power. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sum frequency generation; Mid-infrared detection; Quantum cascade laser
1. Introduction For a long time non-linear up-conversion of light has been used for imaging [1,2], for infrared spectroscopy [3,4] and more recently for photon counting [5]. Non-linear processes, such as second harmonic generation or difference frequency generation, are commonly used for coherent light generation at wavelengths where few or no *Corresponding author. Tel.:+41-22-379-69-36; fax: +41-22-379-39-80. E-mail address: [email protected] (K. Karstad). 1 Now at Institut fur . Experimentalphysik, Universit.at Wien, 1090 Wien, Austria. 0143-8166/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2004.05.006
ARTICLE IN PRESS K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
538
sources exist, and it can also help to solve the inverse problem of detecting light at wavelengths where no efficient detectors are available. In this paper, we consider detection of mid-infrared (mid-IR) radiation (2–20 mm) in the context of free space optical communication using quantum cascade lasers as emitters. In any optical communication systems the wavelength of the carrier beam is chosen according to the transmission medium and performance of the detectors. E.g. in optical fiber communication the wavelengths of the transparency windows (1.3 and 1:55 mm) were chosen, which created a demand for fast and reliable light sources, and fast detectors with low-noise characteristics operating at these wavelengths which eventually led to the development of the indium, gallium and arsenide (InGaAs)-based sources and detectors dominating the telecommunication market today. In free space optical communication wavelengths in the mid-IR exhibit favorable characteristics including low scattering and absorption [6], and it is therefore the wavelength of choice for free space optical links. Fast sources in the mid-IR are now readily available as commercial products in the form of quantum cascades lasers (QCL) [7]. Since this is a very recent development there has, up until now, not been a large demand for fast, low noise detectors operating in the mid-IR, meaning that existing solid-state detectors for these wavelengths fall short of silicon (Si) and InGaAs-based detectors in terms of speed, noise and sensitivity. The existing solid-state detectors for mid-IR detection are mainly based on mercury, cadmium and telluride (MCT), they have signal-to-noise ratios (SNR) several orders of magnitude lower than current Si detectors, and they are a lot slower. Thus, to exploit the superior characteristics of Si detectors, we propose in this paper to detect mid-IR radiation indirectly via non-linear up-conversion in the form of sum-frequency generation (SFG) and use Si detectors to detect the up-converted beam. In the remainder of this paper we discuss the theoretical performances of a detection system based on SFG and Si detectors, followed by a presentation of an experiment demonstrating the virtues of our proposed detection scheme, the constraints of the setup and how they affect the results.
2. Conversion efficiencies SFG is a non-linear process where a source beam, with frequency os ; also called signal, is mixed in a non-linear crystal with a higher-frequency pump beam, with frequency op ; in order to produce a third beam who’s frequency oSFG ; is the sum of the pump and signal frequencies. The efficiency of the process is defined by ZSFG ¼
PSFG ¼ ZPp Ps
ð1Þ
were PSFG ; Ps ; and Pp are the optical powers of the sum-frequency, signal, and pump beams, respectively. The efficiency of the conversion depends on the non-linear material coefficient deff and the pump beam power. For ideal beam parameters, determined by optimizing the beam overlap integral [8], and Gaussian beam profile it
K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
539
Table 1 Ideal conversion efficiency ls ðmmÞ
Crystal
L (mm)
ZSFG =Pp ðW1 Þ
4.5 4.5 9.3
PPLN AGS AGS
10 6 6
2:06 102 8:35 103 3:19 103
is given by [8,9] ZSFG ¼
2 aL 32p2 deff e Lh Pp ; e0 cnSFG ls lp lSFG
ð2Þ
where a a quantity related to the absorption coefficients of each beam, i.e. a combined absorption coefficient, L the crystal length, e0 the free space permitivity, c the speed of light, nSFG the refractive index of the SFG beam, ls ; lp ; and lSFG are the source, pump and SFG beam wavelengths, respectively, and h is the beam overlap integral, a function explained in more detail in the given references, with a global maxima that determines the ideal Rayleigh range for a given crystal length. In this experiment we will demonstrate indirect detection of sources with two different wavelengths, 4.5 and 9:3 mm; both mixed with the same pump wavelength of 980 nm: 4500 nm þ 980 nm-805 nm; 9300 nm þ 980 nm-887 nm: The conversion was performed in periodically poled Lithium Niobate (PPLN) and Silver Thiagolate (AGS) bulk crystals. The selection criterias for these crystals were a wide transparency range spanning 800 nm to the mid-IR, and a large non-linear material coefficient. PPLN has the larger non-linear material coefficient, but could only be used with the 4:5 mm source, since it’s transparency range ends at 5 mm: Crystal parameters were calculated with software published by Sandia National Laboratories (SNLO) [10] and they were used to determine the non-linear conversion efficiency for each crystal and source wavelength, assuming ideal beam overlap and Gaussian beam profiles. The results are listed in Table 1 in units of per W of pump beam power.
3. Detectors In optical communication links the noise characteristics of the detector is of great importance. For a given power incident on a detector it determines the SNR and hence the error rate of a communication, and in some cases it also determines the bandwidth (BW) of the communication. The noise characteristics of a detector is summarized in the noise equivalent power (NEP) defined as the incident light power
ARTICLE IN PRESS K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
540
that produces a SNR equal to unity NEPðW=Hz1=2 Þ ¼
Noise currentðA=Hz1=2 Þ : PhotosensitivityðA=WÞ
ð3Þ
For the sake of comparison, we have compiled a list of commercial MCT and Si solid-state detectors, all operating at room temperature. The peak sensitivity wavelength, NEP and maximum BW of each detector are listed in Table 2, together with the corresponding technology, model number and manufacturer. If we now consider a free space optical link where we aim to communicate with a given bit error rate (BER, typically 109 ), and we assume the noise in the detector follows Gaussian distribution, then the SNR is determined by [11] pffiffiffiffiffiffiffiffiffiffiffi! SNR 1 pffiffiffi BER ¼ erf ð4Þ 2 2 2 and the noise-limited BW of the link is determined by Ps pffiffiffiffiffiffiffiffi; SNR ¼ NEP BW
ð5Þ
where Ps is the signal power incident on the detector. In the case of indirect detection the over all NEP of the detector is NEP NEPoa ¼ ZSFG Pp where ZSFG is the conversion efficiency and Pp is the pump power. Given Eqs. (4) and (5), it is straight forward to calculate the noise-limited BW, or the SNR for a given BW for each of the detectors mentioned in this paper, for a given incident power. In the last column of Table 2 we have listed the modified NEP for the two Si detectors based on indirect detection via up-conversion at 9:3 mm with a modest 40 mW pump power. It indicates that, despite the very low conversion efficiency, we are able to make unrestricted use of the Si detectors higher bandwidth, without having to compromise with a lower SNR. E.g. 30 mW from our 9:3 mm source modulated at 700 MHz and detected with the Vigo PEM-L-3, would yield a SNR of 667. This same SNR can be achieved at 7 GHz with the Perkin-Elmer Si APD and non-linear upconversion with as little as 0:5 mW pump power. Of course this is assuming the QCL can be modulated this fast, something we will investigate closer and report at a later date. Table 2 Detector comparison Manufacturer
Technology
Model
lp ðmmÞ
BW (MHz)
NEP ðfW=Hz1=2 Þ
NEPoa ðfW=Hz1=2 Þ
Vigo Systems Vigo Systems Hamamatsu Perkin-Elmer
MCT MCT Si PIN Si APD
PDI-4 PEM-L-3 S5973 C30902S
4 7.5 0.8 0.9
23 700 1000 7000
63 103 1:7 106 1.5 0.86
1:18 104 6:5 103
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541
4. Experiment The setup of the experiment realized here is illustrated in Fig. 1. The output from the 980 nm diode pump laser is pigtailed with a fibre, single mode for 980 nm; and polarization controllers and a polarizing beam splitter (PBS) are used to select the polarization that maximizes the conversion. A Germanium window with R ¼ 33% and T ¼ 95%; for the pump and QCL beams, respectively, is used to combine the two beams. In order to achieve optimum conversion efficiency, the confocal parameter of each beam should satisfy b ¼ 2ZR ¼
L 2:84
where ZR is the Rayleigh range of each beam and L the crystal length. The conversion efficiency starts dropping quite rapidly when this condition is not fulfilled, so it is important to choose a set of lenses that modify the beam parameters so they closely match the given ideal condition. Between the crystal and detector we use a set of filters and a PBS that block the pump and signal beams and only transmit the up-converted signal.The detector is calibrated for the two up-converted beam wavelengths with diode lasers with wavelengths similar to the up-converted beams. As mentioned in Section 3, we used three different sources in this experiment. *
A Fabry–Pe! rot (FP) QCL source at 4:5 mm operating in pulsed mode with duty cycle of a few percent.
Fig. 1. Schematic of the experiment: the laser diode pump (LD) is focused out of the fiber into the crystal with two lenses. It is overlapped on the mid-IR beam using a Germanium window as a dichroic mirror. The pump is filtered with PBS and filters (short-pass and band-pass).
ARTICLE IN PRESS K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
542 *
*
A distribute feedback (DFB) QCL source at 4:5 mm operating in pulsed mode with duty cycle of a few percent. A FP QCL source at 9:3 mm operating both in continuous wave (CW) and direct modulation mode.
The 9:3 mm source is of greatest interest with respect to the discussed application of free space optical communication links since it is believed that it can be modulated reasonably fast, beyond 1 GHz; but this is still under investigation and will be reported at a later date. The spectral output of the to 4:5 mm FP QCL source is shown in Fig. 2. The spectral output of the 9:3 mm was found to be very narrow as is the case with the 4:5 mm DFB source. The up-conversion is mode dependent, and indeed we find that the conversion efficiency for the DFB QCL at 4:5 mm; who’s spectral output is narrower than that of the FP QCL at 4:5 mm; is greater than the conversion efficiency for the FP laser at the same wavelength. The measured conversion efficiency are listed in Table 3 for their respective wavelength and type of crystal where the conversion took place. We find that the best conversion efficiency we can achieve is off by a factor of 50, when compared with the corresponding theoretical value. This can be accounted for by several factors. *
*
The output of the QC laser is astigmatic and does not conform with the ideal Gaussian beam profile. The Germanium window used to combine the pump and signal beam introduces abberations in the signal beam that deviates it further from the ideal beam model. 3.50E-04 Spectometer Measurement by phase-matching
3.00E-04
Power [A.U.]
2.50E-04
2.00E-04
1.50E-04
1.00E-04
5.00E-05
0.00E+00 4350
4400
4450
4500
4550
4600
4650
4700
4750
wavelength [nm] Fig. 2. Spectrum of the Fabry–P!erot quantum cascade laser. The thin line is a measurement done with a spectrometer, the thick line is made by measuring the wavelength and power of the SFG signal for different phase-matching angles.
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543
Table 3 Measured non-linear coefficients ls ðmmÞ
Crystal
ZSFG;measured =Pp ðW1 Þ
4.5 (DFB) 4.5 (DFB) 4.5 (FP) 9.3
PPLN AGS AGS AGS
1:01 104 2:76 105 1:37 106 6:38 105
0.12 0.11 0.10 0.09 0.08 0.07
U[V]
0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 -0.02 0.00E+000
1.00E-008
2.00E-008
3.00E-008
4.00E-008
5.00E-008
t [s] Fig. 3. Osciloscope trace of up-converted beam, source modulated at 60 (lower curve), 150 and 300 MHz (upper curve). *
The lenses available to us at the time of the experiment did not quite match the ideal conditions for our crystals.
In our experiment we achieve an up-converted beam power of around 70 nW; a power that is bright enough to be detected with a Si APD. In our setup we used a detector from Menlo Systems with a bandwidth of 800 MHz: Fig. 3 shows an oscilloscope trace of the up-converted beam detected with a Si APD with the 9:3 mm source modulated at 60, 150 and 300 MHz: The reduction of each signals amplitude is mainly due to the BW of some of the electronics, but this is currently under scrutiny and will be reported at a later date. We find that the up-converted beam is very aberrated when exiting the crystal and we have trouble focusing the beam to a spot size smaller than the detector area, which has a diameter of 0:5 mm: This explains the noisiness of the signals in Fig. 3.
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K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
Even if the conversion efficiency is off by as much as a factor of 50, a SNR of 667 could be achieved at 7 GHz with the Perkin-Elmer Si APD and non-linear upconversion with a modest 25 mW pump power.
5. Conclusion We have presented an alternative way of detecting mid-IR radiation by using nonlinear up-conversion. We calculated the expected conversion efficiency and presented a comparison of detectors that indicate that this technique has great potential to outperform direct detection, both in terms of speed and noise. The measured conversion efficiency presented was off by a factor of 50 compared with the expected efficiency, but we have reason to believe that we have identified the factors that are causing this discrepancy. Despite the lower than expected conversion efficiency, our results strongly suggest that detection via up-conversion will outperform direct detection and it will be possible to utilize the full BW potential of the faster Si detectors, with an acceptable SNR of 667 with very moderate pump power.
Acknowledgements This project was completed with funding from the Swiss National Science Foundation under the NCCR project Quantum Photonics. We also wish to thank Pr. D. Van der Marel for letting us use his fourier transform infrared spectrometer (FTIR) to characterize our sources.
References [1] Boyd RW, Townes CH. An infrared upconverter for astronomical imaging. Appl Phys Lett 1977;31:440–2. [2] Midwinter JE. Image conversion from 1:6 m to the visible in lithium niobate. Appl Phys Lett 1968;12:68–70. [3] Anfinrud PA, Han C-H, Lian T, Hochstrasser RM. Femtosecond infrared spectroscopy: ultrafast photochemistry of iron carbonyls in solution. J Phys Chem 1993;95:574–8. [4] Dougherty TP, Heilweil EJ. Dual-beam subpicosecond broadband infrared spectrometer. Opt Lett 1993;19:129–31. [5] Langrock C, Roussev RV, Fejer MM, Kurz JR. Talk at LEOS, 2003. [6] Blaser S, Hofstetter D, Beck M, Faist J. Free-space optical data link using Peltier-cooled quantum cascade laser. Electron Lett 2001;37:778–80. [7] Faist J, Capasso F, Sivco DL, Sirtori C, Hutchinson AL, Cho AY. Science 1994;264:553. [8] Boyd GD, Kleinman DA. Parametric interaction of focused gaussian light beams. J Appl Phys 1968;39:3597–639. [9] Sutherlan RL. Handbook of nonlinear optics. New York: Dekker;1996. [10] Sandia National Laboratories: www.sandia.gov/imrl/X1118/xxtal.htm [11] Senior JM. Optical fiber communications, principle and practice, 2nd ed. Englewood Cliffs, NJ: Prentice Hall International Series in Optoelectronics; 1992.
Optics and Lasers in Engineering 43 (2005) 537–544
Detection of mid-IR radiation by sum frequency generation for free space optical communication Ketil Karstada,*, Andre! Stefanova,1, Mark Wegmullera, Hugo Zbindena, Nicolas Gisina, Thierry Aellenb, # Mattias Beckb, Je! rome Faistb a
Group of Applied Physics, University of Geneva, 1211 Geneva 4, Switzerland b # # Institute of Physics, University of Neuchatel, 2000 Neuchatel, Switzerland
Received 6 January 2004; received in revised form 5 May 2004; accepted 7 May 2004 Available online 4 August 2004
Abstract We present an experiment where mid-infrared radiation is detected indirectly via the secondorder non-linear process of sum frequency generation. The mid-infrared sources used for the experiment are quantum cascade lasers, and we use a pump wavelength that yields an upconverted wavelength within the detection window of Silicon avalanche photo diodes. Compared with direct detection using state-of-the-art mid-infrared semiconductor detectors, the detection scheme we propose in this paper has the advantages of greater bandwidth and lower noise equivalent power. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sum frequency generation; Mid-infrared detection; Quantum cascade laser
1. Introduction For a long time non-linear up-conversion of light has been used for imaging [1,2], for infrared spectroscopy [3,4] and more recently for photon counting [5]. Non-linear processes, such as second harmonic generation or difference frequency generation, are commonly used for coherent light generation at wavelengths where few or no *Corresponding author. Tel.:+41-22-379-69-36; fax: +41-22-379-39-80. E-mail address: [email protected] (K. Karstad). 1 Now at Institut fur . Experimentalphysik, Universit.at Wien, 1090 Wien, Austria. 0143-8166/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2004.05.006
ARTICLE IN PRESS K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
538
sources exist, and it can also help to solve the inverse problem of detecting light at wavelengths where no efficient detectors are available. In this paper, we consider detection of mid-infrared (mid-IR) radiation (2–20 mm) in the context of free space optical communication using quantum cascade lasers as emitters. In any optical communication systems the wavelength of the carrier beam is chosen according to the transmission medium and performance of the detectors. E.g. in optical fiber communication the wavelengths of the transparency windows (1.3 and 1:55 mm) were chosen, which created a demand for fast and reliable light sources, and fast detectors with low-noise characteristics operating at these wavelengths which eventually led to the development of the indium, gallium and arsenide (InGaAs)-based sources and detectors dominating the telecommunication market today. In free space optical communication wavelengths in the mid-IR exhibit favorable characteristics including low scattering and absorption [6], and it is therefore the wavelength of choice for free space optical links. Fast sources in the mid-IR are now readily available as commercial products in the form of quantum cascades lasers (QCL) [7]. Since this is a very recent development there has, up until now, not been a large demand for fast, low noise detectors operating in the mid-IR, meaning that existing solid-state detectors for these wavelengths fall short of silicon (Si) and InGaAs-based detectors in terms of speed, noise and sensitivity. The existing solid-state detectors for mid-IR detection are mainly based on mercury, cadmium and telluride (MCT), they have signal-to-noise ratios (SNR) several orders of magnitude lower than current Si detectors, and they are a lot slower. Thus, to exploit the superior characteristics of Si detectors, we propose in this paper to detect mid-IR radiation indirectly via non-linear up-conversion in the form of sum-frequency generation (SFG) and use Si detectors to detect the up-converted beam. In the remainder of this paper we discuss the theoretical performances of a detection system based on SFG and Si detectors, followed by a presentation of an experiment demonstrating the virtues of our proposed detection scheme, the constraints of the setup and how they affect the results.
2. Conversion efficiencies SFG is a non-linear process where a source beam, with frequency os ; also called signal, is mixed in a non-linear crystal with a higher-frequency pump beam, with frequency op ; in order to produce a third beam who’s frequency oSFG ; is the sum of the pump and signal frequencies. The efficiency of the process is defined by ZSFG ¼
PSFG ¼ ZPp Ps
ð1Þ
were PSFG ; Ps ; and Pp are the optical powers of the sum-frequency, signal, and pump beams, respectively. The efficiency of the conversion depends on the non-linear material coefficient deff and the pump beam power. For ideal beam parameters, determined by optimizing the beam overlap integral [8], and Gaussian beam profile it
K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
539
Table 1 Ideal conversion efficiency ls ðmmÞ
Crystal
L (mm)
ZSFG =Pp ðW1 Þ
4.5 4.5 9.3
PPLN AGS AGS
10 6 6
2:06 102 8:35 103 3:19 103
is given by [8,9] ZSFG ¼
2 aL 32p2 deff e Lh Pp ; e0 cnSFG ls lp lSFG
ð2Þ
where a a quantity related to the absorption coefficients of each beam, i.e. a combined absorption coefficient, L the crystal length, e0 the free space permitivity, c the speed of light, nSFG the refractive index of the SFG beam, ls ; lp ; and lSFG are the source, pump and SFG beam wavelengths, respectively, and h is the beam overlap integral, a function explained in more detail in the given references, with a global maxima that determines the ideal Rayleigh range for a given crystal length. In this experiment we will demonstrate indirect detection of sources with two different wavelengths, 4.5 and 9:3 mm; both mixed with the same pump wavelength of 980 nm: 4500 nm þ 980 nm-805 nm; 9300 nm þ 980 nm-887 nm: The conversion was performed in periodically poled Lithium Niobate (PPLN) and Silver Thiagolate (AGS) bulk crystals. The selection criterias for these crystals were a wide transparency range spanning 800 nm to the mid-IR, and a large non-linear material coefficient. PPLN has the larger non-linear material coefficient, but could only be used with the 4:5 mm source, since it’s transparency range ends at 5 mm: Crystal parameters were calculated with software published by Sandia National Laboratories (SNLO) [10] and they were used to determine the non-linear conversion efficiency for each crystal and source wavelength, assuming ideal beam overlap and Gaussian beam profiles. The results are listed in Table 1 in units of per W of pump beam power.
3. Detectors In optical communication links the noise characteristics of the detector is of great importance. For a given power incident on a detector it determines the SNR and hence the error rate of a communication, and in some cases it also determines the bandwidth (BW) of the communication. The noise characteristics of a detector is summarized in the noise equivalent power (NEP) defined as the incident light power
ARTICLE IN PRESS K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
540
that produces a SNR equal to unity NEPðW=Hz1=2 Þ ¼
Noise currentðA=Hz1=2 Þ : PhotosensitivityðA=WÞ
ð3Þ
For the sake of comparison, we have compiled a list of commercial MCT and Si solid-state detectors, all operating at room temperature. The peak sensitivity wavelength, NEP and maximum BW of each detector are listed in Table 2, together with the corresponding technology, model number and manufacturer. If we now consider a free space optical link where we aim to communicate with a given bit error rate (BER, typically 109 ), and we assume the noise in the detector follows Gaussian distribution, then the SNR is determined by [11] pffiffiffiffiffiffiffiffiffiffiffi! SNR 1 pffiffiffi BER ¼ erf ð4Þ 2 2 2 and the noise-limited BW of the link is determined by Ps pffiffiffiffiffiffiffiffi; SNR ¼ NEP BW
ð5Þ
where Ps is the signal power incident on the detector. In the case of indirect detection the over all NEP of the detector is NEP NEPoa ¼ ZSFG Pp where ZSFG is the conversion efficiency and Pp is the pump power. Given Eqs. (4) and (5), it is straight forward to calculate the noise-limited BW, or the SNR for a given BW for each of the detectors mentioned in this paper, for a given incident power. In the last column of Table 2 we have listed the modified NEP for the two Si detectors based on indirect detection via up-conversion at 9:3 mm with a modest 40 mW pump power. It indicates that, despite the very low conversion efficiency, we are able to make unrestricted use of the Si detectors higher bandwidth, without having to compromise with a lower SNR. E.g. 30 mW from our 9:3 mm source modulated at 700 MHz and detected with the Vigo PEM-L-3, would yield a SNR of 667. This same SNR can be achieved at 7 GHz with the Perkin-Elmer Si APD and non-linear upconversion with as little as 0:5 mW pump power. Of course this is assuming the QCL can be modulated this fast, something we will investigate closer and report at a later date. Table 2 Detector comparison Manufacturer
Technology
Model
lp ðmmÞ
BW (MHz)
NEP ðfW=Hz1=2 Þ
NEPoa ðfW=Hz1=2 Þ
Vigo Systems Vigo Systems Hamamatsu Perkin-Elmer
MCT MCT Si PIN Si APD
PDI-4 PEM-L-3 S5973 C30902S
4 7.5 0.8 0.9
23 700 1000 7000
63 103 1:7 106 1.5 0.86
1:18 104 6:5 103
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541
4. Experiment The setup of the experiment realized here is illustrated in Fig. 1. The output from the 980 nm diode pump laser is pigtailed with a fibre, single mode for 980 nm; and polarization controllers and a polarizing beam splitter (PBS) are used to select the polarization that maximizes the conversion. A Germanium window with R ¼ 33% and T ¼ 95%; for the pump and QCL beams, respectively, is used to combine the two beams. In order to achieve optimum conversion efficiency, the confocal parameter of each beam should satisfy b ¼ 2ZR ¼
L 2:84
where ZR is the Rayleigh range of each beam and L the crystal length. The conversion efficiency starts dropping quite rapidly when this condition is not fulfilled, so it is important to choose a set of lenses that modify the beam parameters so they closely match the given ideal condition. Between the crystal and detector we use a set of filters and a PBS that block the pump and signal beams and only transmit the up-converted signal.The detector is calibrated for the two up-converted beam wavelengths with diode lasers with wavelengths similar to the up-converted beams. As mentioned in Section 3, we used three different sources in this experiment. *
A Fabry–Pe! rot (FP) QCL source at 4:5 mm operating in pulsed mode with duty cycle of a few percent.
Fig. 1. Schematic of the experiment: the laser diode pump (LD) is focused out of the fiber into the crystal with two lenses. It is overlapped on the mid-IR beam using a Germanium window as a dichroic mirror. The pump is filtered with PBS and filters (short-pass and band-pass).
ARTICLE IN PRESS K. Karstad et al. / Optics and Lasers in Engineering 43 (2005) 537–544
542 *
*
A distribute feedback (DFB) QCL source at 4:5 mm operating in pulsed mode with duty cycle of a few percent. A FP QCL source at 9:3 mm operating both in continuous wave (CW) and direct modulation mode.
The 9:3 mm source is of greatest interest with respect to the discussed application of free space optical communication links since it is believed that it can be modulated reasonably fast, beyond 1 GHz; but this is still under investigation and will be reported at a later date. The spectral output of the to 4:5 mm FP QCL source is shown in Fig. 2. The spectral output of the 9:3 mm was found to be very narrow as is the case with the 4:5 mm DFB source. The up-conversion is mode dependent, and indeed we find that the conversion efficiency for the DFB QCL at 4:5 mm; who’s spectral output is narrower than that of the FP QCL at 4:5 mm; is greater than the conversion efficiency for the FP laser at the same wavelength. The measured conversion efficiency are listed in Table 3 for their respective wavelength and type of crystal where the conversion took place. We find that the best conversion efficiency we can achieve is off by a factor of 50, when compared with the corresponding theoretical value. This can be accounted for by several factors. *
*
The output of the QC laser is astigmatic and does not conform with the ideal Gaussian beam profile. The Germanium window used to combine the pump and signal beam introduces abberations in the signal beam that deviates it further from the ideal beam model. 3.50E-04 Spectometer Measurement by phase-matching
3.00E-04
Power [A.U.]
2.50E-04
2.00E-04
1.50E-04
1.00E-04
5.00E-05
0.00E+00 4350
4400
4450
4500
4550
4600
4650
4700
4750
wavelength [nm] Fig. 2. Spectrum of the Fabry–P!erot quantum cascade laser. The thin line is a measurement done with a spectrometer, the thick line is made by measuring the wavelength and power of the SFG signal for different phase-matching angles.
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Table 3 Measured non-linear coefficients ls ðmmÞ
Crystal
ZSFG;measured =Pp ðW1 Þ
4.5 (DFB) 4.5 (DFB) 4.5 (FP) 9.3
PPLN AGS AGS AGS
1:01 104 2:76 105 1:37 106 6:38 105
0.12 0.11 0.10 0.09 0.08 0.07
U[V]
0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 -0.02 0.00E+000
1.00E-008
2.00E-008
3.00E-008
4.00E-008
5.00E-008
t [s] Fig. 3. Osciloscope trace of up-converted beam, source modulated at 60 (lower curve), 150 and 300 MHz (upper curve). *
The lenses available to us at the time of the experiment did not quite match the ideal conditions for our crystals.
In our experiment we achieve an up-converted beam power of around 70 nW; a power that is bright enough to be detected with a Si APD. In our setup we used a detector from Menlo Systems with a bandwidth of 800 MHz: Fig. 3 shows an oscilloscope trace of the up-converted beam detected with a Si APD with the 9:3 mm source modulated at 60, 150 and 300 MHz: The reduction of each signals amplitude is mainly due to the BW of some of the electronics, but this is currently under scrutiny and will be reported at a later date. We find that the up-converted beam is very aberrated when exiting the crystal and we have trouble focusing the beam to a spot size smaller than the detector area, which has a diameter of 0:5 mm: This explains the noisiness of the signals in Fig. 3.
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Even if the conversion efficiency is off by as much as a factor of 50, a SNR of 667 could be achieved at 7 GHz with the Perkin-Elmer Si APD and non-linear upconversion with a modest 25 mW pump power.
5. Conclusion We have presented an alternative way of detecting mid-IR radiation by using nonlinear up-conversion. We calculated the expected conversion efficiency and presented a comparison of detectors that indicate that this technique has great potential to outperform direct detection, both in terms of speed and noise. The measured conversion efficiency presented was off by a factor of 50 compared with the expected efficiency, but we have reason to believe that we have identified the factors that are causing this discrepancy. Despite the lower than expected conversion efficiency, our results strongly suggest that detection via up-conversion will outperform direct detection and it will be possible to utilize the full BW potential of the faster Si detectors, with an acceptable SNR of 667 with very moderate pump power.
Acknowledgements This project was completed with funding from the Swiss National Science Foundation under the NCCR project Quantum Photonics. We also wish to thank Pr. D. Van der Marel for letting us use his fourier transform infrared spectrometer (FTIR) to characterize our sources.
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