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Abstract
Quantum-enhanced stimulated Raman scattering (QE-SRS) is a promising technique for highly sensitive molecular vibrational imaging and spectroscopy surpassing the shot noise limit. However, the previous demonstrations of QE-SRS utilized rather weak optical power which hinders from competing with the sensitivity of state-of-the-art SRS microscopy and spectroscopy using relatively high-power optical pulses. Here, we demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD) scheme, which works even when using high-power optical pulses. We used 4-ps pulses to generate pulsed squeezed vacuum at a wavelength of 844 nm with a squeezing level of −3.28 ± 0.12 dB generated from a periodically-poled stoichiometric LiTaO3 waveguide. The squeezed vacuum was introduced to an SRS spectrometer employing a high-speed spectral scanner to acquire QE-SRS spectrum in the wavenumber range of 2000–2280 cm-1 within 50 ms. Using SRS pump pulses with an average power of 11.3 mW, we successfully obtained QE-SRS spectrum whose SNR was better than classical SRS with balanced-detection by 2.27 dB.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Squeezed light has played a pivotal role in pushing the sensitivity of various metrology methods beyond the shot noise limited (SNL) sensitivity since it was firstly observed in 1980s [1]. Quantum enhancement of sensitivity with squeezed light has been applied to gravitational-wave detection [2], plasmonic sensing [3], particle tracking [4], frequency-modulation spectroscopy [5], optical imaging [6,7] and microscopy [8,9]. Recently, quantum enhancement for biological measurements based on nonlinear optics has attracted more and more attention, including the applications in two-photon absorption spectroscopy [10], stimulated emission microscopy [11], stimulated Brillouin scattering microscopy [12], and stimulated Raman scattering (SRS) spectroscopy [13] and microscopy [14].
Among these quantum-enhanced nonlinear optical measurements, quantum-enhanced (QE)-SRS is particularly promising because SRS microscopy enables high-speed and label-free molecular vibrational imaging [15–21]. SRS microscopy has been opening up various biomedical applications [22–30], while its sensitivity is limited by the laser shot noise [31–34]. Although QE-SRS helps to improve the sensitivity by reducing the shot noise, it is still challenging to exceed the sensitivity of classical SRS microscopy. This is mainly because of the difficulty in applying high-power squeezed light pulses (either SRS pump or Stokes) in QE-SRS. For example, if amplitude squeezing with a seeded optical parametric amplifier (OPA) is employed for QE-SRS, the optical power is limited by the deamplification in the squeezing process, and therefore the maximum power was up to 3 mW [14]. Another way to realize QE-SRS is to combine squeezed light and a coherent beam with an asymmetric beamsplitter (BS), which leads to a significant loss of the coherent beam, resulting in a low optical power of 1.3 mW [13]. On the other hand, typical average power of laser pulses for SRS imaging is of several tens of milliwatts [15,18]. It is well known that signal-to-noise ratio (SNR) is proportional to the square root of optical power in SNL measurements. In SRS microscopy where modulated Stokes pulses are used for lock-in detection of pump pulses, SNR is proportional to the product of Stokes pulse power and the square root of pump pulse power. Therefore, in order to exceed the sensitivity of classical SRS, it is crucial to develop QE-SRS system that can operate at a high-power regime.
In this paper, we demonstrate SRS spectroscopy with quantum-enhanced balanced detection (QE-BD), which we recently proposed for quantum enhancement in high-power regime [35]. Firstly, we produced a picosecond pulsed squeezed vacuum (SQV) via single-pass OPA in a waveguide [36]. Its squeezing level was measured as −3.28 ± 0.12 dB at the center wavelength of ∼844 nm. Next, the SQV was introduced to an SRS spectrometer to acquire QE-SRS spectrum. Using SRS pump pulses with an average power of 11.3 mW, we successfully obtained QE-SRS spectrum whose SNR was better than classical SRS with balanced-detection by 2.27 dB. We believe that our work is an important step forward towards beating the SNL sensitivity of state-of-the-art SRS microscopy by using quantum enhancement.
2. Methods
2.1 Principle of SRS spectroscopy with QE-BD
The detailed principle of SRS spectroscopy with QE-BD is described in [35]. Briefly, SRS pump pulses at an angular optical frequency of ωp and intensity-modulated Stokes pulses at ωS are prepared for SRS excitation, and pulsed SQV at ωp is generated from the OPA pump pulses. The SRS pump pulses are combined with the pulsed SQV with a BS to generate two beams, whose shot noise is correlated with each other [35]. One of the output beams is combined with the Stokes pulses, and focused on a sample to be measured. At the focus, SRS occurs when the vibrational resonance frequency ωR of molecules matches with ωp − ωS, and transfers the modulation of Stokes pulses to the SRS pump pulses. The amount of intensity modulation is proportional to the product of the intensities of the SRS pump pulses and the Stokes pulses. Then the transmitted beam is collected by another lens and the Stokes pulses are removed by a filter. The remaining SRS pump pulses from the sample and the other output beam from the BS are detected by a balanced detector, which can cancel the shot noise even beyond the shot noise limit. SRS signal can be obtained by demodulating the output of the balanced detector.
Compared with the amplitude squeezing with a seeded OPA [14] or by displacing a squeezed light with a BS [13], QE-BD is advantageous for achieving quantum enhancement in a high-power regime of >10 mW, which is a typical power for SRS microscopy. This is because QE-BD can employ a 50:50 BS, which can have a high transmittance. We note that, in order to achieve quantum enhancement of SNR compared with classical SRS microscopy, a squeezing level higher than 3 dB is necessary because the balanced detection employs two detectors, which increases the shot noise by 3 dB. Nevertheless, QE-BD is potentially advantageous for achieving a high squeezing level when it is limited by the optical loss caused by the sample and focusing optics. This is because the SQV is split by a BS and only one of the output beams is affected by optical loss. A detailed analysis on these points was already described in [35]. In addition, QE-BD can be applied to several methods on balanced-detection SRS spectroscopy and microscopy [33,37–40], where classical intensity noise of SRS pump laser is canceled by the balanced detection. In our work, as a proof-of-principle experiment, we demonstrate that QE-BD can work at a high-power regime.
2.2 Experimental setup
A simplified schematic of SRS spectrometer with QE-BD is shown in Fig. 1. We used a picosecond Ti:sapphire (TiS) laser generating 4 ps optical pulses at a repetition rate of frep = 76.52 MHz and a center wavelength of ∼844 nm (optical path expressed in blue color) as the source of SRS pump pulses. The TiS laser beam was split into two by a half-wave plate (HWP) and a polarizing beamsplitter (PBS). The vertically polarized beam was launched to second-harmonic generator (SHG) to generate OPA pump light at the center wavelength of ∼422 nm (optical path expressed in purple color). And the OPA pump was further used for SQV generation via single-pass OPA. The horizontally polarized beam was used for producing the signal light of OPA and the local oscillator (LO) light for balanced homodyne detection of SQV or balanced heterodyne detection of OPA signal/idler light. The signal light was used only for investigating the nondegenerate OPA performance and was blocked in the squeezing experiment. The frequencies of the signal light and the LO were shifted by two acousto-optic modulators (AOM1 and AOM2), which worked at the +1st order and the −1st order diffraction angles, respectively. They were installed for the convenience of detecting nondegenerate OPA and squeezing signals and for sweeping the LO phase. A spatial light modulator (SLM) was employed for the spatial beam shaping of LO in order to optimize the detection efficiency of SQV. The light coming from the OPA crystal was combined with LO by a 50:50 BS consisting of two PBSs and an HWP. The horizontally polarized beam coming out of the 50:50 BS was directly detected by one of the two photodiodes of a home-made balanced photodetector (BPD), while the vertically polarized beam was used for SRS detection. In practice, the HWP between the two PBSs was rotated by a tiny portion so that the squeezing level in QE-SRS can be maximized.
Fig. 1. A simplified schematic of SRS spectrometer with QE-BD. SHG, second-harmonic generator; OPA, optical parametric amplifier; AOM, acousto-optic modulator; SLM, spatial light modulator; DM, dichroic mirror; SPF, short-pass filter; LPF, long-pass filter; HWP, half-wave plate; PBS, polarizing beamsplitter; BS, beamsplitter; PD, photodiode; YDFA, Yb-doped fiber amplifier; G, grating; GS, galvanometric scanner; RFSA, radio-frequency spectrum analyzer; LIA, lock-in amplifier; OSC, oscilloscope.
The SRS Stokes pulses were generated by a home-made Yb-doped fiber laser (YDFL) mode-locked by the nonlinear polarization rotation, which produces broadband optical pulses at a repetition rate of ∼38.26 MHz and a center wavelength of ∼1030 nm (optical path expressed in red color). The Yb laser beam was split into two, and they were used for laser synchronization and wavelength scanning. For the synchronization of SRS pump and Stokes laser pulses, two beams were spatially combined by a dichroic mirror (DM) and launched to a GaAsP photodiode (G1115, Hamamatsu Photonics K.K.) to obtain the intensity cross-correlation via two-photon absorption. The photodiode signal was used for the feedback control of the repetition rate of YDFL with an intracavity electro-optic modulator and a piezo stage (not shown in the figure) to achieve synchronization [32,41]. For wavelength scanning, we installed an optical band-pass filter consisting of a reflective diffraction grating and a galvanometric scanner in a 4-f system to realize a wavelength tunability of ∼30 nm [18,41,42]. After being amplified by two-stage Yb-doped fiber amplifiers (YDFAs), the Yb beam was spatially combined with the TiS beam by another DM. The combined beams were loosely focused by a lens (f = 50 mm) into a cuvette filled with liquid samples. And the transmitted beams were collected by another lens (f = 50 mm). Finally, SRS pump light was detected by the other photodiode of the BPD to obtain the stimulated Raman loss (SRL) signal. Taking the advantage that our Stokes laser generates pulses at the Nyquist frequency of pump laser, the SRL signal appears as a ∼38.26 MHz intensity modulation on SRS pump light. This SRL signal can be extracted by a lock-in amplifier (LIA) working at the same frequency as the intensity modulation.
2.3 Nonlinear optical crystals
A 3-mm-long bulk crystal of periodically-poled stoichiometric LiTaO3 (PPSLT) (Shimadzu Corp.) and a 10-mm-long PPSLT waveguide (Shimadzu Corp.) were used for SHG and OPA, respectively. They were designed for type-0 quasi-phase-matching (the 3rd order and the 1st order for SHG and OPA, respectively) at a fundamental wavelength of ∼844 nm and a crystal temperature of ∼55°C. The temperatures of the two crystals were adjusted by home-made temperature controllers. In the squeezing experiment, the wavelength of TiS laser and the temperature of crystals were manually optimized so that the squeezing level can be maximized. Figure 2(a) and (b) show the lateral and top views of the ridge-waveguide OPA crystal, respectively. Both of the input and output surfaces of the crystal were cut at an angle of 10 deg. to prevent multiple reflections. The output surface is covered with an anti-reflective (AR) coating at 844 nm to minimize the optical loss of SQV. In our system, a 10 × objective lens (UPLFLN10X2, Olympus Corp.) was used to couple the free-space OPA pump and signal beams into the waveguide while minimizing the axial chromatic aberration. The light coming out of the OPA crystal was collimated by an aspheric lens (C330TMD-B, f = 3.1 mm, NA = 0.70, AR coating in 600–1050 nm, Thorlabs, Inc.).
Fig. 2. (a) The lateral view of the waveguide OPA crystal. Scale bar: 5 µm. (b) The top view of the waveguide OPA crystal. Scale bar: 30 µm. (c) The fundamental mode (transverse electric mode) of the waveguide calculated numerically. Scale bar: 3 µm.
2.4 Spatial beam shaping
The SLM was used for enhancing the spatial mode-matching between SQV and LO as follows. We assumed that the spatial pattern of SQV is the same as the fundamental spatial mode of the PPSLT waveguide because the phase matching is not satisfied for higher order modes. Thus, we calculated the fundamental mode by a waveguide mode-solver (Modesolverpy [43]), assuming the refractive indices of PPSLT and the coating layer as 2.15 and 1.52, respectively. Figure 2(c) shows the calculated fundamental spatial mode of the waveguide. Then we designed and projected a sawtooth wave pattern on the SLM [44] such that it can work as a reflective diffraction grating, whose 1st order diffraction component matches the fundamental mode of the waveguide.
2.5 Balanced photodetectors
In our experiments, we used two types of BPDs. For the characterization of the OPA process, we used a BPD with two Si photodiodes (S3399, Hamamatsu Photonics K.K.) with a quantum efficiency of 88% at 844 nm. They were biased at +/−5 V and connected in series to obtain the difference photocurrent, which was low-pass filtered with a cutoff frequency of ∼20 MHz and amplified. For squeezing experiment and QE-SRS experiment where the high quantum efficiency and low-noise characteristics are crucial, we used a BPD with two custom-made Si photodiodes (S3399-9052, Hamamatsu Photonics K.K.), whose quantum efficiency is 96% at 844 nm. These Si-PDs were biased at +/− 8 V to avoid the saturation of the PDs. Their difference photocurrent was sent to a photodetection circuit equipped with a band-pass filter at ∼38 MHz to sensitively detect SRL signal and a band-elimination filter at ∼76 MHz to avoid the circuit saturation by eliminating the strong photocurrent at the repetition rate of SRS pump pulses. A high load resistance of 2 kΩ was used to have a large signal voltage and reduce the effect of electronic noise.
3. Results
Firstly, we confirmed nondegenerate OPA process by injecting OPA pump light and the frequency-shifted signal light into the OPA crystal. The signal-LO and idler-LO interference beats were detected by the balanced heterodyne detection without installing the sample and lenses for SRS detection. The driving frequencies of AOM1 and AOM2 were set to f1 = 75.92 MHz and f2 = 76.02 MHz. Thus, the signal light and LO light had frequency offsets of f1 − frep = −600 kHz (due to the comb spectrum of TiS) and f1 − f2 = −100 kHz, respectively. Therefore, the signal-LO and idler-LO interference beats appeared at 500 kHz and 700 kHz, respectively. Figure 3(a) shows the radio-frequency (RF) spectrum measured by a RF spectrum analyzer (RFSA) with a resolution bandwidth (RBW) of 1 kHz and a video bandwidth (VBW) of 1 kHz. The data were measured with LO power of 4.7 mW, OPA signal power of 50 µW and OPA pump power of 20 mW (when pump was on). We can clearly see the signal-LO beat and idler-LO beat at around 500 kHz and 700 kHz. Notice that the peak located at ∼600 kHz is presumably the unwanted interference among high-order diffracted light coming from the AOMs. Figure 3(b) shows the dependence of the idler-LO beat on the OPA pump power, indicating that the idler power increases by boosting the power of OPA pump light.
Fig. 3. The nondegenerate OPA operation measured by balanced heterodyne detection. (a) RF spectrum of the output signal of the balanced photodetector with and without OPA pump pulses. Signal-LO and idler-LO beats locate at 500 kHz and 700 kHz, respectively. RBW: 1kHz, VBW: 1 kHz. (b) The dependence of the RF power of idler-LO beat on OPA pump power.
Then we evaluated the squeezing performance by blocking the OPA signal light. The relative phase between LO and SQV was swept by setting the driving frequency difference between AOM1 and AOM2 into 5 Hz. Figure 4(a) shows the acquired squeezed noise, shot noise, and electronic noise traces. The data were taken by RFSA (RBW: 1 MHz, VBW: 100 Hz) with zero-span at 38 MHz. The optical power of LO was 7.5 mW at each photodiode and that of OPA pump light was 20 mW at the input of the OPA crystal. The observed anti-squeezing and squeezing (ASQ/SQ) levels were 8.75 ± 0.06 dB and −3.28 ± 0.12 dB, respectively. The electronic noise level was smaller than the shot noise level by 15 dB, indicating the effectiveness of the high load resistance (2 kΩ) of our BPD. Figure 4(b) shows the dependence of ASQ/SQ levels on the OPA pump power. When the OPA pump power was higher than 15 mW, ASQ level showed a decreasing tendency, indicating the occurrence of the blue light induced infrared absorption (BLIIRA) effect.
Fig. 4. Results of the squeezing experiment. (a) Balanced homodyne detection of squeezed vacuum (red line, squeezed noise), and vacuum (blue line, shot noise) measured by an RFSA. Electronic noise level is shown with black line. RBW: 1 MHz, VBW: 100 Hz, center frequency: 38 MHz. (b) Dependence of anti-squeezing and squeezing levels on OPA pump power.
Next, we introduced TiS and Yb beams into a cuvette containing deuterated dimethyl sulfoxide (d6-DMSO) as the sample for the characterization of our QE-SRS setup. It is known that d6-DMSO has a carbon-deuterium (C-D) stretching mode in the cell silent region, which is recently attracting attention for metabolic imaging and supermultiplex imaging [21,25]. The optical power of SRS pump beam was of 31 mW at the entrance of 50:50 BS and of 11.3 mW at the sample. The optical power of SRS Stokes beam was 69 mW. Figure 5 shows the measured SRS spectra with and without SQV input, acquired by the RFSA (RBW: 1 MHz, VBW: 100 Hz) with the zero-span mode at 38 MHz. The SRS spectrum was collected within 50 ms by setting the driving waveform of the galvanometric scanner in the Yb system to 10-Hz triangle wave. The C-D stretching mode of d6-DMSO around 2130 cm-1 was clearly observed in all three spectra. The subpeak appearing around 2250 cm-1 was weaker than expected because of the wavelength dependence of the intensity of Stokes pulses. As a result, it could only be observed in SNL-SRS and SQ-SRS spectra, but was buried in the anti-squeezed noise. The anti-squeezed or squeezed component of SQV made the noise floor increased by 10.17 dB or reduced by 2.27 dB, respectively. The relatively low squeezing level compared with the squeezing level of −3.28 ± 0.12 dB is predominantly due to the additional optical loss (∼13%) introduced by the sample, cuvette and the focusing and collecting lenses. We note that in this experiment, the phase of LO/SQV was not locked by any feedback control system. Instead, we set the frequency difference between LO and SQV into 0, but applied a phase modulation of 0.1 Hz to LO, through AOM2. Therefore, comparing with the fast scanned Yb wavelength, the noise floor is kept as a constant during one data acquisition of QE-SRS.
Fig. 5. The QE-SRS spectra acquired with RFSA. On SQ-SRS spectrum, 2.27 dB noise reduction was observed. ASQ-SRS, anti-squeezed SRS; SNL-SRS, shot noise limited SRS; SQ-SRS, squeezed SRS. RBW: 1 MHz, VBW: 100 Hz, center frequency: 38 MHz.
Furthermore, to show that our method is compatible with high-speed data acquisition and high-speed imaging, we extracted the 38 MHz SRL signal with a home-made LIA. The repetition rate signal from YDFL was used as the reference signal for LIA. Figure 6(a) shows the measured QE-SRS spectra by observing the output signal of the LIA on an oscilloscope (OSC). Similarly, depending on anti-squeezing or squeezing, the noise became stronger or weaker. To estimate the noise level quantitatively, we plotted the histogram of the voltage signals in Fig. 6(b). By applying Gaussian fitting, the standard deviation σ were derived as 0.378, 0.139, 0.109 V for ASQ-, SNL-, and SQ-SRS traces, respectively. This corresponds to the quantum enhancement of 2.11 dB. Compared with the result that we got on RFSA, the reduction of the quantum enhancement is presumably due to the noise of electronic circuits and can be mitigated by optimizing the circuit design. Note that the difference in SNR between Fig. 5 and Fig. 6 originates from the different detection bandwidths.
Fig. 6. The QE-SRS spectra acquired with a lock-in amplifier and an oscilloscope. (a) The measured SRS spectra with and without SQV. (b) Noise distribution with Gaussian fitting. (σASQ = 0.40 V, σSNL = 0.152 V, σSQ = 0.116 V.)
4. Discussion
In the present system, the squeezing level of −3.28 ± 0.12 dB was observed, and the shot noise of SRS pump pulses was reduced by 2.27 dB in SRS setup, and the resultant SNR of balanced detection SRS was improved as well. Importantly, we succeeded in conducting QE-SRS with 11.3 mW SRS pump pulses, which is higher than previous reports by ∼4 times [13,14]. Thus, our demonstration confirms that the QE-BD works for the quantum enhancement in a high-power regime. The optical power of SRS pump light in the present experimental setup is primarily limited by the optical loss due to the two AOMs, SLM, and other optical elements in our system. By further optimizing the setup, we can increase the power of SRS pump pulses so that higher SNR can be achieved unless the power exceeds the photodamage threshold of SRS samples.
Another advantage of QE-BD over amplitude squeezing is that QE-BD approach is applicable to the SRS systems using laser sources with large excess noise such as pulsed fiber lasers, which require the balanced detection scheme to cancel out the excess noise [33,37–40]. Therefore, QE-SRS can be implemented with such practical yet noisy laser sources.
Note that care has be taken when we compare the sensitivities of SRS system based on QE-BD and amplitude squeezing. In QE-BD, optical pulses with similar power are detected by two photodiodes, and therefore the detected shot noise becomes doubled compared with the case of amplitude squeezing with single-photodiode detection. On the other hand, as already mentioned in Subsection 2.1, QE-BD scheme is more tolerant to optical loss because SQV is divided into two beams by a 50:50 BS, and only one of them passes through the lossy microscope optics and the sample. Thus, when the squeezing level is limited by the lossy optics, QE-BD can provide higher squeezing level than amplitude squeezing, so that the overall sensitivity enhancement factor of SRS setups based on QE-BD and amplitude squeezing becomes comparable [35].
At the moment, there are several factors that are limiting the squeezing level in our system, such as the quantum efficiency of the photodiodes (88% or 96%), electronic noise of the photodetector circuits (−15 dB below the shot noise level, which is equivalent to the transmittance of ∼97%), transmittance of the optics between the OPA crystal and photodiodes (∼90%, if including the optical loss caused by SRS sample and related optics, <80%), the optical loss caused by the waveguide (not measurable) or BLIIRA, and limited visibility between SQV and LO pulses (<90%) due to the difference in spatial amplitude and phase [45], and temporal chirp [46]. Besides, improving the design of OPA waveguide may also be crucial to improve the squeezing performance. We are investigating into these issues to push forward the squeezing level and the sensitivity of QE-SRS detection.
In the future, we are going to conduct SRS microscopy experiments based on QE-BD. Although the spectroscopy experiment is demonstrated in this work, it is straightforward to realize SRS imaging with QE-BD by installing a stage-scanning microscope with low-loss optics [47,48] and a phase control system to maintain the squeezing level for a long time [49].
5. Conclusion
In summary, we have demonstrated pulsed squeezing with a squeezing level of −3.28 ± 0.12 dB and SRS spectroscopy based on QE-BD. By using high-power SRS pump pulses at 11.3 mW, a noise reduction of 2.27 dB was realized compared with conventional balanced detection SRS. We succeeded in sweeping the wavelength of SRS Stokes pulses and acquiring QE-SRS spectrum of d6-DMSO within 50 ms. We verified that QE-BD is powerful in realizing high-power QE-SRS microscopy. We anticipate that our proof-of-principle experiment will pave the way of developing ultrahigh-speed and ultrasensitive SRS systems surpassing state-of-the-art SRS microscopes and open up new possibilities for various biomedical applications.
Funding
Japan Society for the Promotion of Science (JP 21J21611, JP18K18847); Core Research for Evolutional Science and Technology (JPMJCR1872).
Acknowledgements
We acknowledge Prof. K. Goda and Dr. K. Hiramatsu of The University of Tokyo for their support in the initial phase of this research. We also acknowledge Shimadzu Corp. for the fabrication of nonlinear optical crystals. Y.T. is supported by Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.
Disclosures
The authors are the inventors of the patent application of QE-BD scheme.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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