A compact all solid state continuous-wave biphoton source, tunable around 488 nm, for quantum spectroscopic applications based on a frequency doubled diode laser system is presented. Copolarized photon pairs in the fundamental transversal mode could be generated at 976 nm by spontaneous parametric down conversion inside a type-0 quasi phase matched periodically poled lithium niobate waveguide crystal with an efficiency of 8·10-6. A high flux rate greater than 107 photon pairs per second has been achieved at pump powers in the µW range resulting in more than 7·109 photon pairs/s·mW. Further a detailed investigation of the spectral behavior and the flux rate as a function of the detuning from the degenerated case is presented.
©2008 Optical Society of America
Photon pairs, e.g. generated by spontaneous parametric down-conversion (SPDC), are essential for a number of quantum optic experiments like quantum cryptography [1, 2], quantum computation , teleportation  etc. Further biphoton sources based on SPDC have been considered for several applications like quantum correlation metrology  or quantum spectroscopy [6, 7].
However, since the introduction of periodically poled (PP) materials that use the effect of quasi phase matching (QPM)  for efficient SPDC and the availability of such crystals as waveguides , biphoton sources that deliver high flux rates in one transversal mode have become possible. Several biphoton devices based on PP bulk [12–15] and waveguide [16–21] crystals have been reported. A comparison between bulk and waveguide is given in .
Because of the low losses of optical fibers most quantum communication devices are designed at the typical telecommunication wavelengths of 1.3 µm and 1.5 µm [16–20]. Other work, that has been reported, used shorter wavelengths around 800 nm because of the higher sensitivity of the detectors [8, 9, 12–15]. To achieve compactness and high efficiency diode lasers were applied for the pumping of the SPDC process [8, 9, 12–15, 18]. Normalized flux rates of up to 2.9·10-6 photon pairs/s·mW  and SPDC efficiencies of up to 10-6  have been reported.
In most entanglement or correlation experiments type-II phase matching is used to distinguish signal and idler by the polarization state. For quantum spectroscopy of large molecules it is preferable to have signal and idler in the same state of polarization, so that type-I or even type-0 phase matching should be considered . This means that PP crystals using the high nonlinearity of the d33 coefficient can be used which instead of the smaller d24 for type-II QPM.
In this work a compact all solid state biphoton source is presented, which is explicitly designed for quantum spectroscopic applications, like two photon absorption in large fluorescence markers with an excitation wavelength of 488 nm. It is based on a frequency doubled tunable external cavity diode laser (ECDL) and uses a type-0 periodically poled lithium niobate (PPLN) waveguide crystal for SPDC. A high flux rate of more than 7·109 photon pairs/s·mW combined with tunability of the correlated light wavelength and very good beam quality was achieved.
To our knowledge this represents the highest biphoton-flux rate achieved with a diode laser based system.
2. Two-photon absorption using nonclassical light
When classical light is used the two photon absorption (TPA) rate varies quadratically with the photon-flux density ϕ. In this case the arrival of a photon pair is purely random and therefore the probability that two photons hit the sample at the same time is rather low. The two photon cross section depends on the square of the single-photon cross section and the virtual state life time σTPA=σ 2 SPA·τ and is in the order of 1 GM=10-50 cm4·s. That’s why in classical TPA extremely high photon flux densities of e.g. 1025 photons/cm2 are necessary and mostly fs-pulses with tight focusing are used to deliver the high peak powers necessary for the process. But the high power density has the disadvantage that the damage threshold of an organic specimen is reached or that photo bleaching might occur.
By the use of nonclassical light, the entangled two-photon absorption (ETPA) rate is expected as a linear function of the photon-flux density ϕ [24–27]. A descriptive probabilistic model is given in  resulting in an overall two-photon absorption rate of
With the classical two-photon cross section σTPA and the nonclassical two-photon cross section σETPA. The critical photon-flux density is reached when both processes are equal
Below this case the small random TPA process is dominated by the ETPA process.
This was proven by Lee et al.  and the classical TPA cross section of an organic porphyrin dendrimer was determined to be in the range of σTPA,ω≈10-48 cm4·s (at 800 nm), while the ETPA cross section is 31 magnitudes higher with σETPA,ω≈10-17 cm2. This means that the cross section of the ETPA is comparable with the single photon absorption (SPA) cross section obtained with photons at half the wavelength (σSPA,2ω≈10-16cm2 at 400 nm). In this experiment a BBO type-I crystal was pumped with a frequency doubled Ti:Sapphire laser with a pulse duration of 100 fs and 82 MHz repetition rate. At this duty cycle a flux rate of 3.5·105 photon pairs/s was sufficient for ETPA.
3. Experimental setup
The biphoton source described here is based on a frequency doubled external cavity diode laser (ECDL). A gain guided broad area laser (BAL) diode with an anti-reflection (AR) coated front facet is frequency stabilized and bandwidth narrowed by the use of a diffraction grating in a Littrow configuration. An unstable V-shaped external cavity [28–31] design is used for beam shaping of the gain guided BAL (manufactured by M2K-LASER GmbH, Germany), which results in a drastically improvement of the beam quality up to the diffraction limit.
The external cavity consists of only four optical components: the BAL with an emitter width of 400 µm, a fast axis collimator (FAC), a half waveplate (HWP) and a grating with 1800/mm in a Littrow configuration. A detailed description of the unstable V-shaped ECDL can be found in .
Since the emission of the ECDL shows a slight residual ellipticity as well as astigmatism, two crossed cylindrical lenses L1 and L2 are used for the correction of the astigmatism and generation of a beam waist of 20 µm inside a 10 mm bulk PPLN crystal with 5% MgO doping that is made of congruent melt. The bulk crystal was temperature stabilized around room temperature (26°C). A spherical lens L3 is used for collimation of the blue light and the infrared emission is separated from the SH light by the use of a dichroic mirror and a 2 mm BG 18 filter. Behind the filter the blue light is diffraction limited, well collimated and free of astigmatism. The emission wavelength was 487.4 nm and up to 17 mW blue light could be obtained. Because of the high sensitivity of the detectors and the high efficiency of the crystal a set of various NG filters (filters 1) has been used behind the BG 18 for further attenuation of the pump light.
To generate photon pairs by SPDC a 10 mm long type-0 waveguide crystal made of congruent PPLN with 3% MgO doping was used. The waveguide had a size of 3 µm×5 µm (height × width) and was fabricated by proton exchange (HC PHOTONICS). The waveguide crystal was temperature stabilized around 112° C. For the coupling of the generated blue light into the waveguide crystal an aspherical lens L4 with a focal length of 12 mm and an AR coating in the visible was used. The laser to waveguide coupling efficiency was better than 75%. For the collimation of the SPDC light a second aspherical lens L5 with a focal length of 7.5 mm and AR coating in the near infrared was used. A set of three SCHOTT RG 850 short pass filters (filters 2) was used for blocking the blue pump light from the generated light.
The correlated light was coupled into a single mode fiber with an integrated 50:50 beam splitter at 980 nm (THORLABS FC980-50B-FC) that separates signal and idler into the two fiber branches. On the detector side two avalanche photo diodes (PERKIN ELMER SPCM AQR-13) with an efficiency of 15% at this wavelength and a time-correlated single photon counting device (BECKER&HICKL) have been used. Because of the use of a type-0 QPM crystal for SPDC signal and idler have the same polarization and thus no polarization beam splitter could be used.
4. Experimental results
4.1 External cavity diode laser with wavelength tunable SHG
The ECDL yields up to 1 W of diffraction limited, tunable infrared light around a center wavelength of 976 nm with a laser threshold of 0.7 A and a slope efficiency of 0.5 W/A. A bandwidth of 1.8 MHz and a tuning range of more than 35 nm (FWHM) could be achieved .
Figure 2(a) shows the second harmonic output power with a wavelength of 487.4 nm behind the dichroic mirror as a function of the BAL injection current. At an injection current of 2.2 A a maximum output power of 17 mW of diffraction limited blue light was achieved. The output power of the infrared emission of the ECDL as a function of the wavelength is depicted in Fig. 2(b) for an injection current of 2.2 A.
By changing the SHG crystal temperature and simultaneously tilting the grating of the infrared pump ECDL the blue SHG light was tunable over several nanometers. The red curve in Fig. 3 shows the SH-QPM wavelength as a function of the crystal temperature for the bulk PPLN crystal used for the generation of the blue light in the experimental setup.
To pre-estimate the optimal temperature for QPM at the degenerated case of SPDC inside the waveguide crystal this measurement was repeated using the waveguide crystal as SHG crystal (black curve in Fig. 3). The temperature coefficients for both crystals were measured to be 0.04 nm/K. Because both crystals have a different poling period the absolute QPM wavelengths for efficient SHG are different at the same temperatures. The tunability of the SPDC light is separately discussed in more detail in section 4.4.
The beam propagation factor of the generated second harmonic light was measured to be M2<1.1 for both axes using a moving slit technique to determine beam radii by the second order momentum of the intensity distribution, following the ISO 11146 standard .
4.2 Photon correlation measurements
For best possible stability of the SHG process as well as the free space laser to waveguide coupling the ECDL was operated at a fixed injection current of 1.3 A. The temperatures of both crystals (Tbulk=26° C, Twaveguide=112° C, λSHG=487.4 nm) were held constant for the highest SHG efficiency and the degenerated case of SPDC, respectively.
In Fig. 4 the total number of generated photon pairs as a function of the incident pump photons is depicted. The incident number of photons has been varied by changing the transmission of the first batch of filters (filters 1 in Fig. 1). A flux rate of 4.0·107 generated photon pairs per second at 1.2·1013 incident photons per second (P=5.4 µW) has been obtained. The highest SPDC efficiency achieved was more than 8·10-6. A linear dependence between the number of incident photons (or input power respectively) was observed, which was expected. Because of the saturation of the detectors the maximum incident power coupled to the waveguide crystal was limited to 5.4 µW. Since the frequency doubled ECDL is capable of 3000 times higher output powers an estimated maximum number of 1010–1011 photon pairs per second can be generated at the maximum SHG output power of the ECDL.
A coincidence measurement at 2 µW pump power inside the waveguide is shown in Fig. 5. The measuring time was 10 s and a coincidence peak of 2690 and a coincidence count rate of 4.5·104 coincidences per second were determined. The data acquisition range was 50 ns with 4096 data points, the width of the coincidence curve was 0.769 ns (FWHM) and is limited by the electronics of the measurement system.
4.3 Spatial properties of the SPDC light
To investigate the transversal mode quality of the SPDC light an electron multiplying (EM) CCD camera (ANDOR IXON EM+) with 512×512 pixels and a pixel size of 16 µm was used. The generated light was collimated with the aspherical lens L5 and imaged onto the camera. Because of the high sensitivity of the EMCCD additional neutral density filters were used behind the PPLN waveguide crystal (filters 2 in Fig. 1).
Figure 6 shows the intensity distribution of the generated biphotons in the infrared in front of the fiber coupler. The Gaussian fit through the data of the slices obtained with camera data gives an overlap of 99.5% for the x-direction and 99.4% in y-direction. This leads to the assumption, that the beam is diffraction limited. A slight ellipticity was observed which is caused by the different size of the waveguide in x- and y-direction.
4.4 Spectral investigation of the SPDC light
As mentioned in section 4.1 the QPM wavelength in a periodically poled material is a function of the temperature. Thus the wavelength of the nonlinear process using the QPM grating can be changed by adjusting the temperature of the material. Therefore the SPDC emission of our source is tunable in two ways: firstly the wavelength of the degenerated case is tunable, secondly the SPDC emission can be detuned from the degenerated case.
4.4.1 Tuning wavelength of the degenerated SPDC emission
The wavelength for the degenerated SPDC is tunable by tuning the emission wavelength of the frequency doubled ECDL (by changing the bulk crystal temperature and the grating angle simultaneously) and adjusting the QPM grating of the waveguide crystal for the degenerated case of SPDC (by changing the temperature of the waveguide crystal).
Figure 7 shows the SPDC pump wavelength at maximum coincidence count rates as a function of the temperature. Two exemplarily selected SHG emission wavelengths (λSHG1=487.4 nm, λSHG2=488.6 nm) have been investigated. The blue crosses indicate the measured temperatures (Twaveguide1=112° C, Twaveguide2=140° C) of the waveguide crystal with the highest biphoton flux-rates. A third reference measurement was performed using a 488.0 nm Argon Ion laser (Twaveguide3=130° C). The deviation of the determined values from the SHG QPM curve was negligible.
4.4.2 (De-)Tuning the wavelength around the degenerated SPDC
The degenerated case is not necessarily the best for the investigation of ETPA. Since the ambition of this work is to set up a biphoton source for quantum spectroscopy, the SPDC emission wavelength should be tunable around the degenerated case as well. This is possible when the SPDC pump wavelength emitted by the frequency doubled ECDL is fixed and the temperature of the waveguide crystal is changed, only.
For the spectral investigation of this “detuning” the fiber beam splitter (Fig. 1) was replaced by a single mode fiber which was attached to an optical spectrum analyzer (OSA) AQ-A6315A by Ando Electric Co. with a resolution of 50 pm and a broad measuring range.
Moreover, the diode injection current of the ECDL was increased to 2.2 A and the NG 4 filters have been removed. The pump power behind the BG 18 filter was 12 mW which results in 9 mW incident into the waveguide. A coincidence measurement using TCSPCD at this high output power was not possible because the avalanche photo diodes saturated. But an estimation results in a flux-rate of 6.3·1010 photon pairs/s. While the temperature of the SHG crystal remained constant at 26° C (corresponding to λSHG1=487.4 nm), the temperature of the SPDC waveguide crystal was detuned from the degenerated case by a few K.
Figure 8 (a–d) shows the evolution of the SPDC spectrum for four different waveguide (SPDC) crystal temperatures from 114° C to 111° C while the bulk (SHG) crystal was operated at 26° C. At a temperature of 114° C (Fig. 7 (a)) two peaks around 975 nm are observed. One peak is located at 955 nm the other at 995 nm, each with a bandwidth of about 14 nm (FWHM). By decreasing the crystal temperature to 113° C the two peaks merge and the bandwidth is 30 nm (FWHM). For the degenerated case shown in Fig. 7 (c) the bandwidth narrows to 18 nm (FWHM) and by decreasing the temperature further the spectrum broadens to 30 nm again (Fig. 7 (d)). At 115° C the distance between the two peaks was 55 nm centered around 975 nm. Above 115° C no spectral signal could be detected out of the noise. This spectral behavior is a proof that PDC is present at high pump powers, as expected.
The normalized biphoton-flux rate as described in section 4.2 was 7·109 photon pairs/s·mW. Taking into account that this was measured at the degenerated case with a bandwidth of 18 nm (FWHM) this can be further normalized to 3.9·108 photon pairs/s·mW·nm, respectively.
Another common way to characterize a light emitting source is the average number of photon pairs emitted per coherence time 〈n〉=N·τc, with τc=0.44·λ2/(c·Δλ). Considering a measured bandwidth of Δλ=18 nm around a center wavelength of λ=974.8 nm this gives a coherence time of τc=7.76·10-14s. The number of generated photon pairs at 9 mW was estimated to be 6.3·1010 photon pairs/s, which results in 〈n〉=0.0065.
4.4.3 Influence of the detuning around the degenerated SPDC to the biphoton flux-rate
The measured coincidence rate as a function of the crystal temperature is shown in Fig. 9 for a pump wavelength of λ1=487.4 nm. This measurement was performed under the same condition as described in section 4.2, at a low coupled blue power of 2 µW inside the waveguide.
The degenerated case with the highest biphoton flux rate of 4.5·104 coincidences/s was observed at a waveguide temperature of 112° C. For the broadest detected spectrum (at 115°C) the biphoton flux rate was a 4.5 times lower compared to the maximum, but still exceeded 104. Further even at a detuning of 5 nm from the peak emission the detected coincidence rate was measured to be above 103 photons per second.
A compact all solid state biphoton source based on a frequency doubled external-cavity enhanced broad area laser diode is presented. The SHG light is centered around the technically interesting wavelength of 488 nm and is tunable over several nanometers. The SPDC light at 976 nm can be used for correlated two-photon spectroscopy of green fluorescent proteins .
A high biphoton flux rate of more than 107 photon pairs per second is achieved with very low pump powers of a few µW by coupling the SHG light into a 10 mm proton exchange waveguide PPLN crystal with type-0 quasi phase matching. The efficiency of the SPDC was higher than 8·10-6 and a linear dependence between input power and flux rate was observed. By assuming the process to remain linear the maximum flux rate generated by the device is in the order of 1011 biphotons per second. In  a fs-laser with a duty cycle of 8.2·10-5 was used and 3·105 photon pairs/s have been sufficient for ETPA experiments. Considering the duty cycle of a rate of 1010 photon pairs/s should be high enough for correlated two photon absorption experiments with cw light.
Due to the tunability of the frequency doubled ECDL the wavelength of the degenerated SPDC case can be tuned over several nanometers. Further it is possible to operate the SPDC in the non-degenerated case by changing the waveguide crystal temperature. Detailed investigations of the spectral evolution and the influence of non-degeneration on the SPDC efficiency show promising results for the use of this device in quantum spectroscopy applications. The source can be tuned regarding the center wavelength of the degenerated case and by the degree of degeneration while the correlated photons are emitted in the same transversal mode which makes it very flexible. The beam quality of the blue pump light was measured to be M2<1.1 and the SPDC light shows a near Gaussian emission. This results in a high brightness of the biphoton emission. The bandwidth of the SPDC emission was measured to be 18 nm (FWHM). This is relatively narrow compared to pulsed SPDC sources as a result of the use of a narrow band continuous-wave pump laser.
Photorefractive damage and blue induced infrared absorption were not obtained and can thus be excluded. With the same crystal more than 140 mW blue light by SHG have been generated without the observation of any of these processes .
To our knowledge the generated normalized biphoton-flux rate of more than 7·109 photon pairs/s·mW (and 3.9·108 photon pairs/s·mW·nm, respectively) represents the highest values using diode lasers in a SPDC process in this spectral region.
Since the overall efficiency of the device is mainly limited by the efficiency of the bulk PPLN crystal used for the SHG the use of a second PPLN waveguide crystal for the frequency doubling should improve the performance of the system by at least one order of magnitude. Further it is possible to use type-I or type-II QPM waveguide crystals for the SPDC.
The authors would like to thank Benjamin Freyer for helpful discussions about nonclassical two-photon absorption, Dr. Michael Seefeldt for sharing knowledge about fiber coupling and Dirk Heinrich for the construction of the two crystal oven.
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