2.1.

Laser Description

In Refs. 20–23, it has been shown that the energy of short ($\sim 100\mathrm{ps}$) optical pulse emission in gain-switching mode can be markedly increased using a GaAs/GaAlAs quantum well laser diode with a large ratio of $d_{\mathrm{act}}/\mathrm{\Gamma }>2\mu \mathrm{m}$, where $d_{\mathrm{act}}$ and $\mathrm{\Gamma }$ are the active layer thickness and confinement factor, respectively. Here a compact CMOS laser driver fabricated in a 0.35-high-voltage (HV) CMOS technology was used to drive a specially grown quantum well semiconductor laser diode utilizing the enhanced gain-switching mode. The laser diode has a stripe width and length of $7\mu \mathrm{m}$ and 1.5 mm, respectively, and it generates optical pulses with a peak power of $\sim 0.5\mathrm{W}$. The amplitude of the driving current pulse generated by the CMOS laser driver is adjusted with the supply voltage of the driver ($V_{\mathrm{HV}}$). The pulsing rate achieved with the CMOS driver is 1 MHz. For most of the tests presented in the following, $V_{\mathrm{HV}}$ was set to 18 V, which leads to a current pulse amplitude of $\sim 1.3\mathrm{A}$ for about 1 ns. The laser driver (CMOS driver in Fig. 1) is mounted together with the semiconductor laser diode (LD in Fig. 1) and a few other passive discrete components on a small printed circuit board ($12\times 6{\mathrm{mm}}^2$) as shown in Fig. 1(a). In addition, the metal can of the laser transmitter and all needed connections except ground, which is connected at the bottom of the board by using vias, are shown in Figs. 1(b) and 1(a), respectively. More details can be found in Ref. 19. To trigger the light emission at the desired rate, we generated a square wave synchronism signal with 5 V pulses lasting a few tens of nanoseconds at 1-MHz repetition rate.

Fig. 1

(a) A photograph of the laser transmitter and (b) a packaged laser transmitter with a coin for scale.

2.2.

Laser Characterization

In TD diffuse optics, performances in retrieving optical properties of diffusive media and in localizing deep perturbation can be affected by the instrument response function shape, which is given by the convolution of the laser pulse shape, the detector response, and other jitter contributions.¹⁶ Its shape is usually evaluated not only in terms of full-width at half maximum (FWHM, i.e., width at 0.5 of the peak value) but also down to some orders of magnitude below the peak value (i.e., 0.1 or 0.01 of the peak value), where the effect of reflections, bumps, or decay tails can enlarge the response, thus affecting performances. A first step of this work is therefore the evaluation of the laser pulse shape at different supply voltages $V_{\mathrm{HV}}$ of the laser, ranging from 17 to 35 V. The setup used was composed of the laser described earlier, whose optical pulses were collected by a 1-mm silica fiber ($\mathrm{NA}=0.39$, Thorlabs GmbH, Germany) that was connected to the input of a U-bracket. Another 1-mm silica fiber exiting from the U-bracket focused the collected photons onto the active area of a photomultiplier tube (MCP-PMT with S1 photocathode, Hamamatsu Photonics). Due to a fast temporal response ($<35\mathrm{ps}$ FWHM), free from slow tails, this detector guarantees the reconstruction of the generated optical pulse with negligible distortions over a wide dynamic range ($\sim 4$ orders of magnitude). To adjust the power injected in the collection fiber, we used a variable optical attenuator (VOA) (NDC-100C-4, Thorlabs GmbH, Germany) that was inserted in the clear path of the U-bracket. The “start” signal to the TCSPC board (SPC-130, Becker&Hickl GmbH, Germany) was fed by the PMT whereas the “stop” signal was provided by the main synchronism. For each voltage, we recorded the DTOF for 60 repetitions of 1 s each.

In Fig. 2(a), some of the recorded output pulse shapes for $V_{\mathrm{HV}}=18$, 22, 26, 30, and 34 V are reported, corresponding with current pulse amplitudes of 1.27, 1.53, 1.76, 2.00, and 2.22 A, respectively.

Fig. 2

Characterization of the laser: (a) optical pulse shapes at different $V_{\mathrm{HV}}$ values, (b) temporal characteristics, (c) emission spectrum, (d) emission spectrum characteristics, (e) emitted average power at different $V_{\mathrm{HV}}$, and (f) optical pulse stability over time.

It is possible to notice that the higher the voltage (i.e., the larger the current pulse amplitude), the steeper the rising edge of the emitted laser pulse, and the wider the temporal shape of the laser.

The trend is more marked at voltages lower than 25 V. Indeed, looking at Fig. 2(b) (right axis), the center of gravity (CoG, i.e., the first moment) of the recorded curves, computed at different thresholds (0.5, 0.1, and 0.01 of the peak amplitude), after a $V_{\mathrm{HV}}$ of about 25 V reaches an almost constant value (maximum deviation $<2\%$), if data at 27 and 28 V are not taken into account. Indeed, for those $V_{\mathrm{HV}}$, an unexpected behavior, not in line with the overall trend is noticed and in such points a maximum deviation from average value (computed in all other points of the stable region, $V_{\mathrm{HV}}>2.5\mathrm{V}$) is 4.5%. The abnormal discontinuity might be caused by the relaxation oscillation of the semiconductor laser diode, which is a normal behavior when working in the gain-switching mode. At these specific supply voltages, the second peak of the relaxation oscillation has exceeded a certain level, thus increasing the pulse width, and then suddenly dropped because of an electrical disturbance of the driving current of the laser diode. This behavior can be avoided with better transmitter circuit board design.

The variation of the CoG position upon increasing the bias voltage highlights whether the waveform dynamic range is dominated by a smooth rise of the power with time or by decay tails or bumps after the peak power. In particular, from Fig. 2(b), we can see that at $V_{\mathrm{HV}}<20\mathrm{V}$ the position of the CoG is almost independent of the considered threshold, in agreement with the fact that the pulse shape is quite symmetric in the first two-orders of magnitude from the peak. On the contrary, at $V_{\mathrm{HV}}>20\mathrm{V}$, the CoG computed at the threshold of 0.5 is $\sim 100\mathrm{ps}$ lower than those computed at lower thresholds. This is due to the fact that the pulse shape has a steeper rise on the left and a higher tail on the right, which affects the CoG dependence with the threshold.

Upon an increase in the applied voltage, an increase of the width at level 0.5 (i.e., the FWHM) of the temporal curve can be noticed (left axis). Indeed, the FWHM is 122 ps at $V_{\mathrm{HV}}=17\mathrm{V}$ while it reaches 253 ps at $V_{\mathrm{HV}}=35\mathrm{V}$. In all cases, the FWHM is lower than 260 ps. These performances are compatible with TD diffuse optics measurements, where lasers with FWHM of about 100 to 200 ps are typically used.¹¹ Additionally, laser pulses of a few hundred picoseconds width at 0.01 of the peak value have been already demonstrated suitable for the diffuse optics application in a previous work.¹⁶ The $V_{\mathrm{HV}}$-dependent shift in the CoG of the laser shape is not a problem for diffuse optics applications, where the temporal reference is given by the acquisition of the instrument response function (see Ref. 24).

We also analyzed the emission spectrum of the laser as a function of $V_{\mathrm{HV}}$ by means of a spectrometer (OceanOptics, NIRQuest series), as shown in Fig. 2(c). The mean central wavelength is about 797 nm. In Fig. 2(d), both the width of the emission spectrum at different levels from the peak wavelength and the centers of gravity as a function of $V_{\mathrm{HV}}$ are shown. The spectral width increases upon higher $V_{\mathrm{HV}}$, while for the center of gravity there is no clear trend. Indeed, in the range of $V_{\mathrm{HV}}$ between 17 and 20 V, the CoG moves toward shorter wavelengths for increasing $V_{\mathrm{HV}}$, while for $V_{\mathrm{HV}}>20\mathrm{V}$ it shifts toward longer wavelengths. The spectral width (FWHM) of the laser [Fig. 2(d), left axis) is always lower than 8 nm. This value is very close to the one of the state-of-the-art TD spectroscopy setup²⁵ that features a spectral bandwidth of 7 nm at 1350 nm. For this reason, the proposed laser can be considered suitable also for spectroscopy measurements.

Using a power meter (Thorlabs GmbH, calibration: 800 nm), we measured the average power emitted from the laser at the different $V_{\mathrm{HV}}$ values. Results are shown in Fig. 2(e). The measured power has a clear linear behavior, as underlined by the linear fit (black dotted line) that approximates with high accuracy the experimental data ($R^2>0.999$). Due to heat dissipation issues of the driver chip, it was not possible to explore the laser power behavior for $V_{\mathrm{HV}}>34\mathrm{V}$. Even at this low repetition rate (1 MHz), the average power is suitable for diffuse optical measurements, being of the same order of magnitude of the output power of state-of-the-art fiber-based TD clinical systems.²⁶ The measured power consumption at 1 MHz and at $V_{\mathrm{HV}}=18\mathrm{V}$ is 50 mW (160 mW @ $V_{\mathrm{HV}}=34\mathrm{V}$), thus making this source also compatible with battery operation.

Finally, we tested the output power stability of the laser system following the standardized procedures defined by the basic instrumental performances protocol.²⁴ The working point was set at $V_{\mathrm{HV}}=18\mathrm{V}$, where the laser pulse features an FWHM of 100 ps and an average output power of $62\mu \mathrm{W}$. Even if the emitted power is only 12% of the maximum value obtained at $V_{\mathrm{HV}}=34\mathrm{V}$, the narrow pulse (99 ps FWHM) should ensure better performance in recovering optical properties of highly scattering media. In this case, we decided to use an SiPM module as a detector ($1{\mathrm{mm}}^2$ active area, see Ref. 27 for details), which is insensitive to thermal perturbations, thus ensuring the possibility to properly monitor possible fluctuations of the laser optical power. Hence, photons exiting from the laser were coupled using a collimator into a silica fiber (core diameter: 1 mm); a VOA hosted in a U-bracket was used to properly attenuate the power to fit the single-photon statistics;³ attenuated light was collected by another 1-mm-core silica fiber (whose propagation modes were completely filled using a thin layer of Teflon) coupled to the SiPM module due to a doublets of lens performing 1:1 imaging ($f=30\mathrm{mm}$). We recorded the DTOF for about 100 min with an integration period of 10 s.

For each DTOF, the background noise was subtracted and then both the overall number of counts (i.e., area of the curve) and the moments (at 0.5, 0.1, and 0.01 levels) were computed. Results are shown in Fig. 2(f). After an initial warm-up of 20 min, the number of counts and the center of gravity are almost stable (within $\pm 1\%$ and $\pm 10\mathrm{ps}$, respectively), apart from fast changes possibly related to instabilities that could be improved by employing proper heat dissipation strategies.

3.1.

MEDPHOT Protocol

To assess the suitability of the proposed compact setup in retrieving the optical properties—absorption (${\mu }_{\mathrm{a}}$) and reduced scattering coefficient (${\mu }_{\mathrm{s}}^{\prime }$)—of homogeneous phantoms, we adopted the linearity test defined by the MEDPHOT protocol.²⁸ The linearity test consists of the measurement of four series of solid phantoms featuring different reduced scattering coefficients (labeled A, B, C, and D with ${\mu }_{\mathrm{s}}^{\prime }=2.8$, 5.6, 8.5, and $11.27{\mathrm{cm}}^{-1}$ at 800 nm, respectively). Each series is composed of eight phantoms with constant ${\mu }_{\mathrm{s}}^{\prime }$ and covering a wide range of absorption coefficients (labeled from 1 to 8 and spanning ${\mu }_{\mathrm{a}}$ from 0 to $0.32{\mathrm{cm}}^{-1}$ at step of $\sim 0.05{\mathrm{cm}}^{-1}$ at 800 nm).²⁸ Despite the low repetition rate, the high laser power permitted measuring all phantoms in transmittance geometry (thickness of the phantom around 45 mm) and we collected the DTOF with 60 repetitions of 1 s each.

To obtain the values of ${\mu }_{\mathrm{a}}$ and ${\mu }_{\mathrm{s}}^{\prime }$, the measured DTOFs were fitted using an analytical solution of the radiative transport equation under the diffusion approximation,²⁹ convoluted with the instrument response function (previously measured, data not shown).

Results are shown in Fig. 3 where measured ${\mu }_{\mathrm{a}}$ and ${\mu }_{\mathrm{s}}^{\prime }$ are shown against their conventionally true values for the kit of phantoms employed.²⁸ The first graph [Fig. 3(a)] shows the linearity in recovering the absorption coefficient of the phantoms against their conventionally true values. The behavior is linear for all phantoms series (minimum $R^2=0.9977$ for phantoms with ${\mu }_{\mathrm{s}}^{\prime }=2.8{\mathrm{cm}}^{-1}$). For all of the series, the slope of the fit is only a few percent higher than expected (from worst case: C-series with a recovered slope of 1.16 instead of 1, thus leading to an error of 16%).

Fig. 3

Results from MEDPHOT protocol. (a) Displays absorption linearity, (b) the coupling of the absorption coefficient to the scattering one, (c) the scattering linearity, (d) the coupling of the scattering coefficient to the absorption one. For all measurements the laser driver was operating at $V_{\mathrm{HV}}$ of 18 V.

Figure 3(b) shows the dependence of the recovered ${\mu }_{\mathrm{a}}$ on the conventionally true ${\mu }_{\mathrm{s}}^{\prime }$, thus highlighting a possible scattering-to-absorption coupling, while Fig. 3(c) enlightens the absorption-to-scattering coupling. For both cases, ideally, a horizontal line should be displayed, meaning that there is no cross talk between ${\mu }_{\mathrm{a}}$ and ${\mu }_{\mathrm{s}}^{\prime }$. As expected, the coupling is higher for higher absorption due to deviation from the diffusion approximation.

The last graph in Fig. 3(d) reports the linearity for the recovered reduced scattering coefficient displaying overall a good linearity (with $R^2>0.9895$). Apart from the “null” absorption case with a slope of 1.32 instead of 1 due to some criticalities in dealing with very low absorption, for the other seven series the discrepancy in slope is always $<15\%$.

3.2.

nEUROPt Protocol

With the same setup described in Sec. 3.1, we tested the capability to detect a perturbation buried inside a homogeneous background. For this purpose, we used the “contrast” (C) and “contrast-to-noise ratio” (CNR) tests defined in the “nEUROPt protocol.”³⁰ For its implementation, we used a switchable solid phantom (described in Ref. 31) that allows one to move an absorbing perturbation along one direction. The optical properties of the background phantom were ${\mu }_{\mathrm{a}}=0.1{\mathrm{cm}}^{-1}$ and ${\mu }_{\mathrm{s}}^{\prime }=8{\mathrm{cm}}^{-1}$ while the perturbation features a $\mathrm{\Delta }{\mu }_{\mathrm{a}}$ of $0.17{\mathrm{cm}}^{-1}$ in a volume of $1{\mathrm{cm}}^3$.

Measurements were done in reflectance geometry (source–detector distance: 3 cm) with the inclusion in the center between the laser and the detector. The perturbation was moved in depth, from 5 to 35 mm (steps of 5 mm) with respect to the surface of the phantom. We computed contrast and CNR in time windows (i.e., subdividing the whole DTOF curve into portions) as defined in the nEUROPt protocol.³⁰ In particular, the contrast was calculated summing up all the counts acquired within selected time windows ($t_w$) of the DTOFs for each position of the inclusion ($z$) using

For CNR, we made use of the definition given in Ref. 30

Fig. 4

(a) Contrast and (b) CNR computed for different temporal windows with a perturbation $\mathrm{\Delta }{\mu }_{\mathrm{a}}$ of $0.17{\mathrm{cm}}^{-1}$ in a volume of $1{\mathrm{cm}}^3$.

As expected: (i) the contrast decreases upon increasing depth of the inclusion due to the broadening of the sensitivity profile and (ii) deeper inclusions ($z>5\mathrm{mm}$) are more visible at longer delays of the time window ($>500\mathrm{ps}$) due to the longer path-length of late-arrival photons.

Considering a $\mathrm{CNR}=1$ as a threshold for detectability of a perturbation, the perturbation can be detected at depths up to 25 mm (with time window starting at 2000 ps: $C=0.03$ and $\mathrm{CNR}=1.77$ in Fig. 3). Notwithstanding the compactness of both the laser system and the detector, these values are comparable to other bulkier state-of-the-art systems.¹³